Python: Fix HTTP call to MCP losing tracing parent (#3623)

[alliscode]

Add OpenTelemetry trace context propagation to MCPStreamableHTTPTool's httpx client via an event hook that injects W3C traceparent/tracestate headers into outgoing HTTP requests. This ensures the remote MCP server's spans are correctly parented under the agent framework's Execute Tool span.

• Add _inject_otel_context async event hook using opentelemetry.propagate.inject
• Add _get_instrumented_httpx_client to ensure all httpx clients have the hook
• Cache auto-created httpx clients; clean up in close()
• Update and add tests for trace context injection

Contribution Checklist

• The code builds clean without any errors or warnings
• The PR follows the Contribution Guidelines
• All unit tests pass, and I have added new tests where possible
• Is this a breaking change? If yes, add "[BREAKING]" prefix to the title of the PR.

[markwallace-microsoft]

Python Test Coverage Report •

File	Stmts	Miss	Cover	Missing
packages/core/agent_framework
_mcp.py	424	66	84%	72, 74, 117–118, 128–133, 144, 149, 195–196, 206–211, 221–222, 274, 283, 346, 354, 375, 489, 556, 591, 593, 597–598, 600–601, 655, 670, 688, 729, 835, 848–853, 875, 921–922, 928–930, 949, 974–975, 979–983, 1000–1004, 1148
TOTAL	20494	3217	84%

Python Unit Test Overview

Tests	Skipped	Failures	Errors	Time
3945	225 💤	0 ❌	0 🔥	1m 9s ⏱️