fix(classifier): treat Anthropic "out of extra usage" 400 as billing

[charleneleong-ai]

Anthropic returns HTTP 400 with "You're out of extra usage. Add more at claude.ai/settings/usage and keep going." when a user's extra-usage allowance is depleted. This wording is not in _BILLING_PATTERNS in agent/error_classifier.py, so classify_api_error falls through _classify_400 to the generic format_error path (retryable=False, should_fallback=False). The agent aborts instead of engaging the configured fallback chain (fallback_providers / fallback_model).

Fix: add "out of extra usage" to _BILLING_PATTERNS. Now classified as FailoverReason.billing → should_fallback=True, matching how other providers' billing-exhaustion messages are handled.

Repro

Primary Anthropic model with depleted extra usage + Gemini fallback configured:

⚠️  API call failed (attempt 1/3): BadRequestError [HTTP 400]
   🔌 Provider: anthropic  Model: claude-opus-4-6
   📝 Error: HTTP 400: You're out of extra usage. Add more at claude.ai/settings/usage and keep going.
❌ Non-retryable client error (HTTP 400). Aborting.

After the fix the same error triggers the fallback chain and the agent continues on the fallback provider.

Test plan

• Added test_400_anthropic_extra_usage_exhausted in tests/agent/test_error_classifier.py asserting reason == billing, should_fallback is True, retryable is False.
• pytest tests/agent/test_error_classifier.py — 98 passed.
• Verified locally: after restarting the gateway with the patch, the same 400 advances to the configured fallback provider instead of aborting.

[trevorgordon981]

LGTM. Clean, focused fix. The new pattern matches the shape of other entries in _BILLING_PATTERNS and the test pins the classification contract (reason, fallback, retryable) explicitly.

[alt-glitch]

Related to #13170 — identical fix (add 'out of extra usage' to _BILLING_PATTERNS). This PR and #13170 are competing implementations.

[teknium1]

Fixed and merged via PR #56128 (rebase-merged, commit ea9e8d6 — your authorship preserved in git history).

"out of extra usage" (Anthropic OAuth Pro/Max overage bucket depleted) was misclassifying as a generic format_error — non-retryable with no credential rotation. It's now in _BILLING_PATTERNS, so it classifies as billing: non-retryable, rotates credential, falls back, and surfaces Anthropic-specific guidance (claude.ai/settings/usage + cycle-reset).

You were the earliest of four independent PRs submitting this exact fix — thanks for catching it first.