fix(error-classifier): treat flaky free-model gateway 403/Forbidden as transient

[anicca-earn]

What

Franklin's SOL-chain free-model gateway (sol.blockrun.ai) proxies through an upstream (e.g. NVIDIA NIM) whose credential is flaky — roughly 1 in 3 calls comes back 403 Forbidden Authorization failed even though the gateway's own auth is fine, and the very same request typically succeeds on retry.

classifyAgentError() in src/agent/error-classifier.ts had no branch matching 403. Neither the 401/unauthorized auth branch nor the 500-504 server branch matched it, so the error fell through to the catch-all unknown category with isTransient: false. Since the agent loop's recovery logic (src/agent/loop.ts) only retries when isTransient is true, every session that hit this upstream hiccup died outright instead of retrying past a transient blip.

Fix

Add a branch that classifies "authorization failed" wording (and the general 403 + "forbidden" shape) as category 'server' with isTransient: true. This is deliberately not folded into the existing 'auth' category — 'auth' tells the user to reconfigure their own API key, which isn't the right remedy here (there's nothing wrong with the user's credentials; the gateway's upstream hiccuped). Reusing 'server' means the fix gets the existing transient-retry budget for free, plus loop.ts's server-error streak guard, which automatically switches to the next model in the routing fallback chain if the same model keeps failing — exactly the graceful-degradation behavior a flaky free-model gateway needs, with no new plumbing required.

Testing

Added error classifier treats flaky gateway 403 as transient, not as a user auth failure to test/local.mjs, covering:

• HTTP 403: Forbidden Authorization failed (the llm.ts-style prefixed form)
• API error: 403 Forbidden Authorization failed (the exact wording observed in production)
• HTTP 403: Forbidden (bare form, no echoed "Authorization failed")
• a real 401 Unauthorized: invalid API key still classifies as non-transient auth — confirming this fix doesn't loosen the existing auth-failure path

Ran the full local suite (npm test: 348/348 pass) and npm run test:strategies (3/3 pass) — no regressions. Live/paid suites (test:e2e, test:free-models) were not run since they require network access and real spend, unrelated to this classifier-only change.

Root cause repro (before fix)

classifyAgentError('API error: 403 Forbidden Authorization failed')
// → { category: 'unknown', isTransient: false }   (before)
// → { category: 'server',  isTransient: true  }   (after)