Python: Allow AzureOpenAIResponsesClient creation with Foundry project endpoint (#3814)

* Initial plan

* feat: extend AzureOpenAIResponsesClient to support Foundry project endpoints

Add project_client and project_endpoint parameters to allow creating
the client via an Azure AI Foundry project. When provided, the client
uses AIProjectClient.get_openai_client() to obtain the OpenAI client.
The azure-ai-projects package is imported lazily and only required
when using the project endpoint path.

Co-authored-by: eavanvalkenburg <13749212+eavanvalkenburg@users.noreply.github.com>

* fix: address code review - remove duplicate MagicMock imports in tests

Co-authored-by: eavanvalkenburg <13749212+eavanvalkenburg@users.noreply.github.com>

* fix: add type field to Responses API input items and add Foundry sample

- Add 'type: message' to input items in _prepare_message_for_openai
  to comply with the Responses API schema requirement
- Filter out empty dicts from unsupported content types to prevent
  sending items with invalid empty type values
- Add azure_responses_client_with_foundry.py sample demonstrating
  AzureOpenAIResponsesClient with project_endpoint
- Update README and pyrightconfig.samples.json accordingly

* updates to response format and setup

* fix: patch AIProjectClient at correct module path in test

Patch agent_framework.azure._responses_client.AIProjectClient instead of
azure.ai.projects.aio.AIProjectClient since the import is at module level.

* docs: add Foundry sample to READMEs and document AZURE_AI_PROJECT_ENDPOINT env var

---------

Co-authored-by: copilot-swe-agent[bot] <198982749+Copilot@users.noreply.github.com>
Co-authored-by: eavanvalkenburg <13749212+eavanvalkenburg@users.noreply.github.com>
Co-authored-by: eavanvalkenburg <github@vanvalkenburg.eu>

python/pyrightconfig.samples.json

@@ -5,7 +5,8 @@
         "**/autogen-migration/**",
         "**/semantic-kernel-migration/**",
         "**/demos/**",
-        "**/agent_with_foundry_tracing.py"
+        "**/agent_with_foundry_tracing.py",
+        "**/azure_responses_client_with_foundry.py"
     ],
     "typeCheckingMode": "off",
     "reportMissingImports": "error",