Hold the reentrancy flag while degrading in the fail-safe path

Addresses Copilot review on PR #662.

The fail-safe catch inside prompt delegated to the wrapped prompt WITHOUT
setting $Global:__ShellInteg_Rendering. The normal path clears that flag in its
finally before the catch runs, so an adopted CHAINING prompt calls back into us
with the flag false and the guard does not fire.

Reproduced with a chaining prompt plus a forced failure in the render path:

    'CHAIN|CHAIN|CHAIN| ... (130+ levels) ... PS C:\Users\yeelam> '

That is the exact unbounded mutual-delegation cycle the guard exists to
prevent; only the engine's call-depth limit stopped it. After the fix the same
repro returns 'CHAIN|BASE> ' in 12ms, and the base prompt text is preserved.

Two smaller review points in the same commit:

- The test took script.substr(rearmFunc) without first proving rearmFunc is not
  npos, so a missing marker would surface as std::out_of_range instead of a
  clear VERIFY failure. Added an explicit VERIFY_ARE_NOT_EQUAL.
- check-spelling: 'scriptblocks' is not a recognized word - reworded to
  'script blocks'.

RendersFailSafe now asserts the flag is set before the degraded delegation and
cleared in finally; mutation-tested by removing the assignment, which fails the
test as intended.

104/104 ShellIntegrationTests pass. Behavioural suite still green on Windows
PowerShell 5.1 and PowerShell 7, and the script still survives Constrained
Language Mode on both.

Author: yeelam-gordon

src/cascadia/UnitTests_TerminalCore/ShellIntegrationTests.cpp

@@ -515,9 +515,11 @@ void ShellIntegrationTests::PowerShell_ScriptContent_ReArmsFromReadLineBoundary(
     // Detection is by object identity against our own scriptblock — never by
     // text matching, which a renderer could accidentally satisfy.
     const auto rearmFunc = script.find("function Global:__ShellInteg_Rearm");
-    // -eq, NOT [object]::ReferenceEquals: re-arm runs at script load and from the
-    // readline boundary, and a static method call under Constrained Language Mode
-    // aborts the enclosing script even from inside try/catch.
+    VERIFY_ARE_NOT_EQUAL(std::string::npos, rearmFunc,
+                         L"__ShellInteg_Rearm must exist");
+    // -eq, NOT a static reference-equality method: re-arm runs at script load and
+    // from the readline boundary, and a static method call under Constrained
+    // Language Mode aborts the enclosing script even from inside try/catch.
     const auto identityCheck = script.find("if ($current -eq $Global:__ShellInteg_Wrapper) { return }", rearmFunc);
     const auto noStaticCall = script.substr(rearmFunc).find("[object]::ReferenceEquals");
     const auto adopt = script.find("$Global:__ShellInteg_OriginalPrompt = $current", identityCheck);
@@ -628,15 +630,19 @@ void ShellIntegrationTests::PowerShell_ScriptContent_RendersFailSafe()
     const auto renderTry = script.find("try {", promptStart);
     const auto normalReturn = script.find("return \"${prefix}${originalOutput}${suffix}\"", renderTry);
     const auto renderCatch = script.find("catch {", normalReturn);
-    const auto degradeToPrompt = script.find("try { return (& $Global:__ShellInteg_OriginalPrompt) }", renderCatch);
+    const auto failSafeFlag = script.find("$Global:__ShellInteg_Rendering = $true", renderCatch);
+    const auto degradeToPrompt = script.find("try { return (& $Global:__ShellInteg_OriginalPrompt) }", failSafeFlag);
     const auto lastResort = script.find("catch { return \"PS $($executionContext.SessionState.Path.CurrentLocation)> \" }", degradeToPrompt);
+    const auto failSafeClear = script.find("finally { $Global:__ShellInteg_Rendering = $false }", lastResort);
 
-    VERIFY_ARE_NOT_EQUAL(std::string::npos, lastResort);
+    VERIFY_ARE_NOT_EQUAL(std::string::npos, failSafeClear);
     VERIFY_IS_TRUE(renderTry < normalReturn &&
                        normalReturn < renderCatch &&
-                       renderCatch < degradeToPrompt &&
-                       degradeToPrompt < lastResort,
-                   L"Rendering must fall back to the wrapped prompt, then to a plain prompt, instead of throwing");
+                       renderCatch < failSafeFlag &&
+                       failSafeFlag < degradeToPrompt &&
+                       degradeToPrompt < lastResort &&
+                       lastResort < failSafeClear,
+                   L"The fail-safe must hold the re-entrancy flag while degrading, or an adopted chaining prompt recurses to the call-depth limit");
 }
 
 // ─── Install ──────────────────────────────────────────────────────────────────

src/cascadia/inc/PowerShellShellIntegration.h

@@ -588,8 +588,16 @@ if (-not $Global:__ShellInteg_Installed) {
             # FAIL SAFE. Shell integration is a convenience; a broken prompt is
             # not. Degrade to the wrapped prompt without marks rather than
             # throwing on every prompt.
+            #
+            # The re-entrancy flag must be held here too. The normal path clears
+            # it in its finally before this catch runs, so an adopted CHAINING
+            # prompt would call back into us with the flag false and recurse
+            # until the engine's call-depth limit - the very cycle the guard
+            # above exists to prevent.
+            $Global:__ShellInteg_Rendering = $true
             try { return (& $Global:__ShellInteg_OriginalPrompt) }
             catch { return "PS $($executionContext.SessionState.Path.CurrentLocation)> " }
+            finally { $Global:__ShellInteg_Rendering = $false }
         }
     }
 
@@ -619,7 +627,7 @@ function Global:__ShellInteg_Rearm {
     # call raises "Method invocation is supported only on core types", which
     # aborts the enclosing script even from inside try/catch - breaking profile
     # sourcing or the input path. ScriptBlock does not override Equals, so -eq is
-    # reference identity here (verified: two scriptblocks with identical text
+    # reference identity here (verified: two script blocks with identical text
     # compare False), and the operator is permitted in Constrained Language Mode.
     # That keeps re-arming working there instead of disabling it.
     $current = $function:prompt