Updated pretest routine to not overwrite the testbed metadata. (sonic-net#21834)

Needed for the  switch+dpu pretest combo runs.

Modified the test_pretest.py file to update the testbed metadata on the subsequent calls.
This resolved the pretest conflicts between the switch and the dpu pretests.

What is the motivation for this PR?
Enable pretest for both switch and dpu in any order/place of the test sequence.

How did you do it?
Modified the code to update JSON instead of overwriting it

How did you verify/test it?
Ran multiple switch and DPI regressions

Signed-off-by: nmirin <nikolay.a.mirin@gmail.com>

Author: nikamirrr

tests/test_pretest.py

@@ -142,6 +142,41 @@ def collect_dut_info(dut, metadata):
     metadata[dut.hostname] = dut_info
 
 
+def update_testbed_metadata(metadata, tbname, filepath):
+    """Update or create testbed metadata JSON file.
+
+    Reads existing metadata file (if present), updates or adds testbed metadata,
+    and writes back to file. Handles missing files and JSON decode errors gracefully.
+
+    Args:
+        metadata: Dictionary containing DUT metadata to be stored.
+        tbname: Testbed name used as key in the metadata file.
+        filepath: Path to the metadata JSON file.
+
+    Returns:
+        None.
+    """
+    try:
+        with open(filepath, 'r') as yf:
+            info = json.load(yf)
+        try:
+            info[tbname].update(metadata)
+        except KeyError:
+            logger.info(f"The testbed '{tbname}' is not in the file '{filepath}', adding it.")
+            info[tbname] = metadata
+    except FileNotFoundError:
+        logger.info(f"The testbed metadata file '{filepath}' was not found, creating new file.")
+        info = {tbname: metadata}
+    except json.JSONDecodeError as e:
+        logger.warning(f"Error: Failed to decode JSON from the file '{filepath}': {e}, recreating the file.")
+        info = {tbname: metadata}
+    try:
+        with open(filepath, 'w') as yf:
+            json.dump(info, yf, indent=4)
+    except IOError as e:
+        logger.warning('Unable to create file {}: {}'.format(filepath, e))
+
+
 def test_update_testbed_metadata(duthosts, tbinfo, fanouthosts):
     metadata = {}
     tbname = tbinfo['conf-name']
@@ -151,17 +186,11 @@ def test_update_testbed_metadata(duthosts, tbinfo, fanouthosts):
         for duthost in duthosts:
             executor.submit(collect_dut_info, duthost, metadata)
 
-    info = {tbname: metadata}
     folder = 'metadata'
+    if not os.path.exists(folder):
+        os.mkdir(folder)
     filepath = os.path.join(folder, tbname + '.json')
-    try:
-        if not os.path.exists(folder):
-            os.mkdir(folder)
-        with open(filepath, 'w') as yf:
-            json.dump(info, yf, indent=4)
-    except IOError as e:
-        logger.warning('Unable to create file {}: {}'.format(filepath, e))
-
+    update_testbed_metadata(metadata, tbname, filepath)
     prepare_autonegtest_params(duthosts, fanouthosts)