fix(server): BearerTokenAuthMiddleware now populates ctx.auth_info for bearer flows

[bokelley]

Summary

• auth_context_factory now constructs AuthInfo(kind="bearer", principal=..., credential=None) and stores it at metadata["adcp.auth_info"] when the request is authenticated, so RequestContext.auth_info is non-None for bearer flows — the same typed surface signed-request flows already populate
• _build_ctx pops adcp.auth_info from tool_ctx.metadata after extraction so the AuthInfo object never reaches RequestContext.metadata or the context-echo/idempotency path (mirrors the existing adcp.buyer_agent pop)
• Two new regression tests + one tightened assertion in test_auth_middleware.py

Root cause

auth_context_factory populated caller_identity and tenant_id on the ToolContext but never set metadata["adcp.auth_info"]. PlatformHandler._extract_auth_info reads that key — so for every bearer-authenticated request it silently returned None, leaving ctx.auth_info unset even though ctx.auth_principal was populated (fixed in #574).

Test plan

• pytest tests/test_auth_middleware.py — 38/38 pass
• pytest tests/ — 4092 pass (excluding pre-existing network flake test_real_tls_handshake_still_validates_hostname)
• ruff check src/ — clean
• mypy src/adcp/ — clean
• Security review: bearer token value never stored in AuthInfo, object popped before context-echo path, credential=None suppresses synthesis
• Code review: approved; test nit resolved (disambiguation between discovery-tool and non-discovery unauthenticated paths)

Closes #576

https://claude.ai/code/session_01R254Wmibw6fxiDDTvhuw9b

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