docs(powershell): correct the header-collection note in Get-FalconRegionHeader

The note claimed the success path yields "a Dictionary, so ContainsKey". That was
an assumption. Measured on Windows PowerShell 5.1.26100.9168: Invoke-WebRequest
returns a body-less 3xx as Microsoft.PowerShell.Commands.WebResponseObject, whose
Content is a Byte[] — which is precisely why the old code failed inside
ConvertFrom-Json before it could reach the region logic.

The note now distinguishes what was measured on each platform and path from what
rests on documentation, and no longer names a type nothing verified.

Comment only; no behaviour change. The functional test still passes on
PowerShell 7.6.5 and on 5.1.

Author: carlosmmatos-cs

powershell/install/falcon_windows_install.ps1

@@ -207,16 +207,23 @@ begin {
     }
 
     function Get-FalconRegionHeader($Response) {
-        # Reads X-Cs-Region off a response whose header collection type varies by
-        # platform and by which code path produced it. The wrong accessor fails
-        # quietly rather than loudly, so probe instead of assuming. Measured:
-        #   PowerShell 7 catch      System.Net.Http.Headers.HttpResponseHeaders
-        #                           Contains/GetValues work. The string indexer
-        #                           returns EMPTY instead of throwing, so it must
-        #                           not be tried first.
-        #   Windows PowerShell 5.1  System.Net.WebHeaderCollection, which has a
-        #                           string indexer and no Contains method.
-        #   success path            a Dictionary, so ContainsKey.
+        # Reads X-Cs-Region off a response whose header collection type differs by
+        # platform and by which code path produced it, and where the wrong accessor
+        # fails quietly rather than loudly. Measured:
+        #   PowerShell 7, thrown 3xx    System.Net.Http.Headers.HttpResponseHeaders.
+        #                               Contains/GetValues work, but the string
+        #                               indexer returns EMPTY rather than failing,
+        #                               so it must not be tried first.
+        #   Windows PowerShell 5.1,     Invoke-WebRequest RETURNS the 3xx as a
+        #   returned 3xx                WebResponseObject; this reads the region off
+        #                               it correctly end to end. Its Content is a
+        #                               Byte[], which is what made the old code fail
+        #                               in ConvertFrom-Json before ever getting here.
+        #   5.1, thrown errors          System.Net.WebHeaderCollection, which has a
+        #                               string indexer and no Contains method. The
+        #                               indexer branch below is verified against a
+        #                               real one on 5.1.
+        # Hence: probe by type and by which accessor exists, rather than assuming.
         if (!$Response) {
             return $null
         }

powershell/install/falcon_windows_uninstall.ps1

@@ -193,16 +193,23 @@ begin {
     }
 
     function Get-FalconRegionHeader($Response) {
-        # Reads X-Cs-Region off a response whose header collection type varies by
-        # platform and by which code path produced it. The wrong accessor fails
-        # quietly rather than loudly, so probe instead of assuming. Measured:
-        #   PowerShell 7 catch      System.Net.Http.Headers.HttpResponseHeaders
-        #                           Contains/GetValues work. The string indexer
-        #                           returns EMPTY instead of throwing, so it must
-        #                           not be tried first.
-        #   Windows PowerShell 5.1  System.Net.WebHeaderCollection, which has a
-        #                           string indexer and no Contains method.
-        #   success path            a Dictionary, so ContainsKey.
+        # Reads X-Cs-Region off a response whose header collection type differs by
+        # platform and by which code path produced it, and where the wrong accessor
+        # fails quietly rather than loudly. Measured:
+        #   PowerShell 7, thrown 3xx    System.Net.Http.Headers.HttpResponseHeaders.
+        #                               Contains/GetValues work, but the string
+        #                               indexer returns EMPTY rather than failing,
+        #                               so it must not be tried first.
+        #   Windows PowerShell 5.1,     Invoke-WebRequest RETURNS the 3xx as a
+        #   returned 3xx                WebResponseObject; this reads the region off
+        #                               it correctly end to end. Its Content is a
+        #                               Byte[], which is what made the old code fail
+        #                               in ConvertFrom-Json before ever getting here.
+        #   5.1, thrown errors          System.Net.WebHeaderCollection, which has a
+        #                               string indexer and no Contains method. The
+        #                               indexer branch below is verified against a
+        #                               real one on 5.1.
+        # Hence: probe by type and by which accessor exists, rather than assuming.
         if (!$Response) {
             return $null
         }

powershell/migrate/falcon_windows_migrate.ps1

@@ -1004,16 +1004,23 @@ function Get-FalconCloud ([string] $xCsRegion) {
 
 
 function Get-FalconRegionHeader($Response) {
-    # Reads X-Cs-Region off a response whose header collection type varies by
-    # platform and by which code path produced it. The wrong accessor fails
-    # quietly rather than loudly, so probe instead of assuming. Measured:
-    #   PowerShell 7 catch      System.Net.Http.Headers.HttpResponseHeaders
-    #                           Contains/GetValues work. The string indexer
-    #                           returns EMPTY instead of throwing, so it must
-    #                           not be tried first.
-    #   Windows PowerShell 5.1  System.Net.WebHeaderCollection, which has a
-    #                           string indexer and no Contains method.
-    #   success path            a Dictionary, so ContainsKey.
+    # Reads X-Cs-Region off a response whose header collection type differs by
+    # platform and by which code path produced it, and where the wrong accessor
+    # fails quietly rather than loudly. Measured:
+    #   PowerShell 7, thrown 3xx    System.Net.Http.Headers.HttpResponseHeaders.
+    #                               Contains/GetValues work, but the string indexer
+    #                               returns EMPTY rather than failing, so it must
+    #                               not be tried first.
+    #   Windows PowerShell 5.1,     Invoke-WebRequest RETURNS the 3xx as a
+    #   returned 3xx                WebResponseObject; this reads the region off it
+    #                               correctly end to end. Its Content is a Byte[],
+    #                               which is what made the old code fail in
+    #                               ConvertFrom-Json before ever getting here.
+    #   5.1, thrown errors          System.Net.WebHeaderCollection, which has a
+    #                               string indexer and no Contains method. The
+    #                               indexer branch below is verified against a real
+    #                               one on 5.1.
+    # Hence: probe by type and by which accessor exists, rather than assuming.
     if (!$Response) {
         return $null
     }