#5515 fix: solve #5513 — back off /api/auth/token mints on a broken session

Root cause

JwtApiAuthProvider.getJwt() (host-service) mints a JWT by exchanging the session token at /api/auth/token. On failure it simply threw without caching anything, and there was no in-flight coalescing. Because the cloud tRPC client calls getHeaders() → getJwt() on every request (and invalidateCache() on 401), a broken session turned every request into a fresh network mint with no backoff. A wedged multi-workspace client can generate hundreds of POST /api/auth/token calls per session (the issue reports ~600, ~300× the healthy baseline).

That storm is what tripped Vercel's per-IP rate mitigation (x-vercel-mitigated: deny), which then 403'd the login endpoint behind the same blanket rule — a self-locking loop where the human couldn't re-authenticate to clear the broken state.

This PR addresses item #1 from the issue (client-side backoff / circuit breaker). Items #2 (surfacing x-vercel-mitigated: deny as a real UI error) and #3 (server-side firewall scoping of /api/auth/desktop/connect) are out of scope here.

The fix

In JwtApiAuthProvider:

• Circuit breaker — after a failed mint, refuse to touch the network again until an exponentially growing cooldown elapses (1s → 2s → 4s … capped at 5 min). While the cooldown is active getJwt() rejects immediately with the last error instead of re-hitting /api/auth/token. A success resets the backoff.
• In-flight coalescing — concurrent callers on a cold cache now share a single mint instead of fanning out N token exchanges.

This collapses a broken-session burst from hundreds of requests to a slow trickle, well under any rate-based mitigation.

Tests

JwtAuthProvider.test.ts:

• Reproduction/resolution — 50 sequential failing getHeaders() calls now issue ≤3 network fetches (was 50, one per call).
• Coalescing — 10 concurrent failing calls share a single in-flight fetch (was 10).
• All 8 pre-existing auth-provider tests still pass; typecheck and lint clean.

Closes #5513

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Summary by cubic

Add a circuit breaker and in-flight coalescing to JwtApiAuthProvider to stop repeated /api/auth/token mints when a session is broken. Addresses #5513 by backing off failed mints to avoid rate-mitigated lockouts.

• Bug Fixes
  • Circuit breaker with exponential backoff (1s → 2s → 4s … capped at 5 min); during cooldown getJwt() fails fast with the last error; success resets.
  • Coalesces concurrent getJwt() calls so only one network mint runs on a cold cache.
  • Tests: 50 sequential failures perform ≤3 fetches; 10 concurrent failures share 1 fetch; existing tests still pass.