fix(agentgateway): guard MCP session handshake with anyio.fail_after

[cassiofariasmachado]

Disclaimer: Do not include SAP-internal or customer-specific information in this PR (e.g. internal system URLs, customer names, tenant IDs, or confidential configurations). This is a public repository.

Description

When an AGW MCP endpoint returns HTTP 200 with Content-Type: text/event-stream but a plain-text error body (e.g. "Jwt issuer is not configured"), list_mcp_tools hangs indefinitely. The httpx-sse SSEDecoder silently discards all non-SSE lines, so no events are ever yielded, nothing is written to the response stream, and session.initialize() / session.list_tools() block forever.

Wraps the MCP session handshake (initialize + list_tools) in both list_server_tools (_lob.py) and _list_server_tools (_customer.py) with anyio.fail_after(timeout), reusing the existing timeout parameter from ClientConfig (default 60 s). On expiry, TimeoutError is raised and caught by the asyncio.gather(return_exceptions=True) callers, which log it and continue.

Related Issue

Related to #348

Type of Change

• Bug fix (non-breaking change that fixes an issue)

How to Test

1. Point an AGW MCP destination at an endpoint that returns HTTP 200 with Content-Type: text/event-stream and a plain-text body (e.g. a misconfigured JWT issuer).
2. Call client.list_mcp_tools(...) — previously hangs forever, now raises TimeoutError after the configured timeout.
3. Run unit tests: uv run pytest tests/agentgateway/unit/ -v — all 257 pass.

Checklist

• I have read the Contributing Guidelines
• I have verified that my changes solve the issue
• I have added/updated automated tests to cover my changes
• All tests pass locally
• I have verified that my code follows the Code Guidelines
• I have updated documentation (if applicable)
• I have added type hints for all public APIs
• My code does not contain sensitive information (credentials, tokens, etc.)
• I have followed Conventional Commits for commit messages

Additional Notes

anyio is already a transitive dependency via mcp — no new dependencies introduced.