Make MCP OAuth attempts atomic and eliminate orphan auth sessions

[dcramer]

Problem

Junior eagerly persists an MCP auth session whenever an MCP provider is activated, before knowing whether interaction is required. Successful already-authorized calls leave those records behind for 24 hours.

The per-user/provider index keeps stale IDs and can grow indefinitely for an active user. Same-session link reuse can also delete the live auth session needed by the already-delivered callback URL.

Root causes

• auth attempts are inferred from loosely coupled session/index/pending-auth records
• lookup uses "latest session for user/provider" rather than deterministic attempt identity
• attempt state is created before the SDK reaches an interactive redirect
• link reuse deletes the newly prepared session without proving it is distinct from the reused attempt
• callback claim/completion is not an atomic, idempotent transition

Acceptance criteria

• Silent authenticated MCP calls create no authorization-attempt record.
• Attempts are created lazily only when interaction is required.
• Attempt identity is deterministic from the owning conversation/session/provider or represented by an explicit opaque attempt ID.
• Reusing a live link cannot delete or invalidate its attempt state.
• Attempt indexes prune expired entries and cannot grow indefinitely.
• Callback claim and completion are atomic and duplicate callbacks are idempotent.
• Persisted attempt context includes source, destination, actor, scope, resource, issuer, redirect URI, and resume identity.
• Denied, expired, failed, superseded, and completed attempts all reach terminal states.

Related: #782.

[sentry-pierre]

createMcpOAuthClientProvider in packages/junior/src/chat/mcp/oauth.ts writes the auth session before the SDK decides whether a redirect is needed, so silent already-authorized calls still append to the index.

getLatestMcpAuthSessionForUserProvider picks the latest by updatedAtMs, and putMcpAuthSession resets the index TTL while appending, so the per-user/provider index grows without pruning stale IDs.

finalizeMcpAuthorization has no atomic claim step, so duplicate callbacks would both run transport.finishAuth.

All root causes line up with the code.