Fix flaky integration tests and re-enable skipped tests

- Foundry agent: add allow_preview=True to custom client test
- Foundry hosting: raise max_output_tokens 50->200, add temperature,
  relax assertion in test_temperature_and_max_tokens
- Foundry embedding: update skip reason with root cause (endpoint mismatch)
- OpenAI file search: fix vector store indexing race condition by polling
  file_counts before querying; fix get_streaming_response -> get_response(stream=True)
- Azure OpenAI file search: remove skip (transient 500 resolved)

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

python/packages/foundry/tests/foundry/test_foundry_agent.py

@@ -634,7 +634,6 @@ async def test_foundry_agent_configure_azure_monitor_import_error() -> None:
 @pytest.mark.flaky
 @pytest.mark.integration
 @skip_if_foundry_agent_integration_tests_disabled
-@pytest.mark.skip(reason="Test agent seems to have disappeared from the test environment; needs investigation.")
 async def test_foundry_agent_basic_run() -> None:
     """Smoke-test FoundryAgent against a real configured agent."""
     async with FoundryAgent(credential=AzureCliCredential(), allow_preview=True) as agent:
@@ -648,10 +647,11 @@ async def test_foundry_agent_basic_run() -> None:
 @pytest.mark.flaky
 @pytest.mark.integration
 @skip_if_foundry_agent_integration_tests_disabled
-@pytest.mark.skip(reason="Test agent seems to have disappeared from the test environment; needs investigation.")
 async def test_foundry_agent_custom_client_run() -> None:
     """Smoke-test FoundryAgent against a real configured agent."""
-    async with FoundryAgent(credential=AzureCliCredential(), client_type=RawFoundryAgentChatClient) as agent:
+    async with FoundryAgent(
+        credential=AzureCliCredential(), client_type=RawFoundryAgentChatClient, allow_preview=True
+    ) as agent:
         response = await agent.run("Please respond with exactly: 'This is a response test.'")
 
     assert isinstance(response, AgentResponse)