* Fix MCPStreamableHTTPTool auth headers not being passed correctly

* code review

* code review

* Add unit tests for httpx client configuration in MCP tool registration

Tests verify that:
- Authorization header is set on httpx.AsyncClient
- User-Agent header is set on httpx.AsyncClient
- MCP_HTTP_CLIENT_TIMEOUT_SECONDS constant (90s) is used
- MCPStreamableHTTPTool receives http_client param (not headers)
- httpx clients are tracked in _http_clients for cleanup
- cleanup() properly closes all httpx clients

These tests prevent regression to the bug where passing headers
directly to MCPStreamableHTTPTool was silently ignored.

* Add end-to-end httpx client lifecycle tests

New TestHttpxClientLifecycle class with 4 tests:
- test_full_client_lifecycle_single_server: verifies client created by
  add_tool_servers_to_agent() is properly closed by cleanup()
- test_full_client_lifecycle_multiple_servers: verifies all clients are
  tracked and cleaned up when multiple MCP servers are configured
- test_cleanup_idempotent_no_clients: verifies cleanup() is safe when
  no clients exist
- test_cleanup_called_twice_after_creating_clients: verifies calling
  cleanup() multiple times doesn't cause issues

These tests ensure connection/file-descriptor leaks are prevented by
verifying the full client lifecycle from creation to cleanup.

* Add httpx as runtime dependency for agentframework tooling extension

httpx is now used directly in mcp_tool_registration_service.py to create
AsyncClient instances with pre-configured headers. Without this explicit
dependency, consumers could hit ModuleNotFoundError if httpx isn't
available through transitive dependencies.

* Fix linting issues in MCP tool registration tests

- Remove unused httpx import
- Remove unused mock_mcp_tool variable assignments
- Sort imports properly
- Remove trailing whitespace from blank lines

* Apply ruff formatting to test file

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Co-authored-by: Josina Joy <josjoy@microsoft.com>