feat(auth): persist oauth_metadata via TokenStorage to fix refresh after restart

[mangibaud33]

Summary

OAuthClientProvider._initialize() restores current_tokens and client_info from storage but not oauth_metadata. After a restart with cached tokens, _refresh_token() loses the authoritative token endpoint discovered during the prior session and falls back to <base_url>/token — wrong for servers whose token endpoint sits on a non-standard path.

This PR makes metadata persistence symmetric with tokens/client_info: optional get_oauth_metadata / set_oauth_metadata methods on TokenStorage, loaded in _initialize, saved after OASM discovery.

Closes part of #1318 (Bug 2 — the _refresh_token fallback URL issue). Bug 1 from #1318 (token_expiry_time not restored) is tracked separately in #1784.

Repro

Concrete case: HubSpot MCP Auth App (mcp.hubspot.com). Its discovery document reports "token_endpoint": "https://mcp.hubspot.com/oauth/v3/token". After completing the initial OAuth flow and storing tokens to disk, a restart causes the next refresh attempt to hit https://mcp.hubspot.com/token (the <base_url>/token fallback), which returns 404 HTML. The httpx.Response.json() validation then fails, _handle_refresh_response returns False, and the SDK cascades into a full interactive OAuth flow. That flow cannot complete in non-interactive environments (daemons, containers, CI).

Fix approach

Alternative considered: fetch discovery inline in _initialize via a private httpx.AsyncClient. Rejected — inconsistent with the yield-based flow, complicates testing (would need to mock HTTP in _initialize tests), and can't easily use the same TLS / proxy configuration the host httpx client is using.

Chosen: persist oauth_metadata alongside tokens/client_info. Symmetric with existing storage pattern, zero extra HTTP call on hot path, cache survives restarts.

Changes

src/mcp/client/auth/oauth2.py

• TokenStorage Protocol: add two methods with default no-op bodies (return None / return). Marked as optional in docstrings.
• _initialize: load oauth_metadata via getattr(storage, 'get_oauth_metadata', None) — preserves backward compatibility with duck-typed storage implementations predating this API (they return None and the refresh path falls back to the legacy behaviour).
• async_auth_flow 401 branch: after successful OASM discovery, persist via getattr(storage, 'set_oauth_metadata', None).
• Removed three obsolete # pragma: no cover markers on lines now exercised by the new tests (_initialize, async_auth_flow._initialize call, async_auth_flow except Exception), per AGENTS.md.

tests/client/test_auth.py

5 new tests:

Test	Purpose
test_initialize_restores_oauth_metadata	Happy path: _initialize reads metadata from storage into context
test_refresh_token_uses_persisted_metadata_endpoint	The actual bug: after restart with persisted metadata, _refresh_token uses the correct non-standard endpoint
test_initialize_backward_compat_without_metadata_methods	Duck-typed legacy storage (no get_oauth_metadata) doesn't raise AttributeError on init
test_token_storage_protocol_default_metadata_methods	Protocol defaults return None / no-op for subclasses choosing not to override
test_auth_flow_discovery_with_legacy_storage_skips_metadata_persistence	Full OAuth flow with legacy storage: OASM discovery completes, getattr fallback skips persistence silently

MockTokenStorage also updated to implement the two new methods.

Backward compatibility

• Protocol Subclasses: inherit new default no-op implementations automatically. No action required.
• Duck-typed implementations (matching the 4 required methods without inheriting the Protocol class — the existing MockTokenStorage pattern): continue to work thanks to the getattr fallback in the provider. They will not persist metadata and will re-trigger discovery on each restart, identical to pre-PR behaviour.

Zero breaking change for any existing TokenStorage implementation.

Test plan

• uv run --frozen pytest tests/client/test_auth.py — 101 passed
• uv run --frozen pyright src/mcp/client/auth/oauth2.py tests/client/test_auth.py — 0 errors
• uv run --frozen ruff format + check — clean
• coverage report --include='src/mcp/client/auth/oauth2.py' — 99.38% (parity with main baseline; the only missing line is the pre-existing return data, headers in prepare_token_auth, covered by other test files in CI)
• UV_FROZEN=1 uv run --frozen strict-no-cover — ✅ (removed 3 obsolete pragmas)
• Verified locally against a running HubSpot MCP Auth App server: gateway reconnect no longer triggers browser OAuth flow.

Notes for reviewers

• Happy to backport to v1.x on request (small diff, no API break).
• Prior partial PR on the related token_expiry_time bug: Fix token_expiry_time upon context initialization for stored tokens. #1784. I've commented there offering to help finalize if @keurcien has bandwidth; otherwise I can follow up with a separate rebased PR once this one lands.