Prevent hook headers from overwriting SDK-managed headers

Hook-provided headers now only set headers that aren't already present,
preventing hooks from silently overwriting Authorization, OData, or
correlation headers.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: tpellissier

src/PowerPlatform/Dataverse/data/_odata.py

@@ -235,10 +235,12 @@ def _request(self, method: str, url: str, *, expected: tuple[int, ...] = _DEFAUL
         operation = _OPERATION_NAME.get() or "http.request"
         table_name = _OPERATION_TABLE.get()
 
-        # Merge any hook-provided headers
+        # Merge hook-provided headers without overwriting SDK-managed headers
         hook_headers = self._telemetry.get_additional_headers()
         if hook_headers:
-            request_context.headers.update(hook_headers)
+            for name, value in hook_headers.items():
+                if name not in request_context.headers:
+                    request_context.headers[name] = value
             request_context.kwargs["headers"] = request_context.headers
 
         with self._telemetry.trace_request(