chore(providers): retire Felo Web on provenance hold (#11698)

Merged via /merge-batch (v3.8.51 provenance sweep). Boarded and validated together with the batch's other retirement PRs in a combined worktree — full gate suite green. This PR's conflicts (against #11720's Designer retirement, both introducing a retirement-guard mechanism across executors/index.ts, executorProxy.ts, providers.ts, reservedProviderPrefixes.ts, auth.ts, chat.ts, chatHelpers.ts, model.ts) were reconciled by combining both guards at every chokepoint, with the shared reserved-prefix count recomputed (not guessed) at 400. Re-validated with this PR's own 72 node:test + 20 vitest focused tests, all passing, and pushed before merge. Thank you.

examples/quickstart/python_requests.py

@@ -16,7 +16,7 @@ headers = {
 }
 
 data = {
-    "model": "felo/auto",  # Keyless, works out of the box — no sign-up needed
+    "model": "auto",  # Zero-config routing, works out of the box — no sign-up needed
     "stream": False,
     "messages": [
         {"role": "user", "content": "Hello! What can you do?"}