fix(mcp): refresh expired OAuth tokens automatically

[chengyongru]

Summary

• persist absolute OAuth token expiry, authorization-server metadata, and the issuer bound to newly issued credentials so refresh survives gateway restarts
• refresh an expired token before the request, or discover the same bound authorization server after a 401 and refresh once before interactive authorization
• serialize refreshes per MCP server across providers/processes, then atomically compare-and-swap rotated tokens
• preserve credentials on transient refresh failures; clear only rejected grants/clients and recover from stale token endpoints
• never send legacy unbound or issuer-mismatched refresh tokens to a newly advertised authorization server; those records require one interactive authorization to establish the binding
• activate the gateway-owned MCP connection before cleaning up the temporary OAuth probe, so slow cleanup cannot leave the WebUI in a failed state after successful authorization

Context

Addresses NAN-72.

This is a compatibility layer for the MCP Python SDK restart behavior tracked in modelcontextprotocol/python-sdk#3250 and the proposed upstream fix #3328. An explicit contract test makes incompatible SDK changes fail loudly until the workaround can be removed. Removal after the upstream fix lands is tracked in NAN-73.

Current verification (a91c356)

• Python OAuth and WebUI API tests: 57 passed on MCP SDK 1.30.0; 24 OAuth tests also passed on 1.26.0, including SSE message POST 401 refresh and discovery-outage recovery both in-process and after restart
• WebUI OAuth tests: 11 passed
• WebUI production build, Ruff, and BasedPyright: passed
• Built WebUI on an isolated gateway: MCP catalog reload and failure-to-management interaction passed; the failed credential state used an HTTP response fixture
• Live third-party OAuth was not rerun; external catalog icons returned 404 or timed out
• GitHub Actions: 7 jobs passed, 1 conditionally skipped

Previous verification (73140e2)

• deterministic post-authorization activation regression with blocked probe cleanup: passed
• focused OAuth, MCP, WebUI, and connection suite: 51 passed
• live Linear browser OAuth, actual tool call, forced-401 refresh, and cold-restart persistence: passed
• WebUI production build and isolated Gateway/Apps browser smoke: passed; 0 console errors
• Ruff on changed files: passed
• BasedPyright on changed files: 0 errors
• GitHub Actions: 8/8 jobs passed