Case-insensitive header merge, remove redundant install instruction

- Hook header merge now uses case-insensitive comparison to prevent
  hooks from bypassing SDK headers via different casing
- Remove redundant 'pip install opentelemetry-sdk' from demo
  (already included in [telemetry] extra)

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

Author: tpellissier

examples/telemetry_demo.py

@@ -11,7 +11,6 @@
 
 To run with full OTel:
     pip install "PowerPlatform-Dataverse-Client[telemetry]"
-    pip install opentelemetry-sdk
     python examples/telemetry_demo.py
 
 To run Jaeger locally:

src/PowerPlatform/Dataverse/data/_odata.py

@@ -236,11 +236,14 @@ def _request(self, method: str, url: str, *, expected: tuple[int, ...] = _DEFAUL
         table_name = _OPERATION_TABLE.get()
 
         # Merge hook-provided headers without overwriting SDK-managed headers
+        # (case-insensitive comparison since HTTP headers are case-insensitive)
         hook_headers = self._telemetry.get_additional_headers()
         if hook_headers:
+            existing_lower = {h.lower() for h in request_context.headers}
             for name, value in hook_headers.items():
-                if name not in request_context.headers:
+                if name.lower() not in existing_lower:
                     request_context.headers[name] = value
+                    existing_lower.add(name.lower())
             request_context.kwargs["headers"] = request_context.headers
 
         with self._telemetry.trace_request(