fix(adk): stop leaking Authorization headers in MCP tool error responses

[voltagebots]

What

Reworks ConnectionSafeMcpTool error handling so failure responses and logs never stringify an exception that may carry request metadata:

• _connection_error_response no longer interpolates str(error) into the model-visible message and no longer passes exc_info=error to the logger; it reports only the exception type.
• New _extract_http_status walks the exception chain and any BaseExceptionGroup (via __cause__/__context__/.exceptions, with cycle protection) to find an httpx.HTTPStatusError without stringifying it, and surfaces a sanitized HTTP-status message.
• run_async now handles bare httpx.HTTPStatusError, McpError wrapping an HTTP status, and BaseExceptionGroup wrapping an HTTP status.

Why

An MCP transport failure could previously place str(error) — which may include the outbound Authorization header — into both the LLM-visible error text and the logs. This is a credential-leak path. The fix keeps error responses actionable (type name / HTTP status / "do not retry") while removing the leak surface.

Verified against the pinned transport (mcp==1.29.0)

The credential-leak sanitization is proven by tests to hold regardless of how the error is wrapped (bare HTTPStatusError, McpError-wrapped, ExceptionGroup-wrapped, or a raw ConnectError carrying an Authorization header) — in every case the secret and header name are absent from both the returned dict and the logged output. The connection-error classification itself was checked against the actual pinned mcp package source: streamable_http's POST handler runs inside an anyio task group, which propagates a non-2xx raise_for_status() failure as a BaseExceptionGroup wrapping httpx.HTTPStatusError — exactly the shape the group-walking branch above handles.

Testing

uv run pytest packages/kagent-adk/tests/unittests/test_mcp_connection_error_handling.py — 16/16 passed (python 3.13.15), including all 4 new tests and all 12 pre-existing tests with zero regressions.