fix(ai,coding-agent): deliver Anthropic error status and recover a native tool-search 400 on the same model

[code-yeongyu]

Summary

Fixes the remaining native tool-search 400 cluster:

• Anthropic native tool-search: a real HTTP 400 never reaches after_provider_response, so the adapter's permanent-400 fallback is unreachable #1481 — a real Anthropic HTTP failure never reached onResponse/after_provider_response, because the SDK rejects HTTP errors instead of returning a Response and the only callback ran on success. The native tool-search adapter's permanent 400 fallback was dead code on the live error path.
• Anthropic native tool-search 400 does not recover the failed turn; the session falls back to another model instead of retrying without native injection #1482 — after a native tool-search 400 the session's hard-error branch always switched to the next fallback model, demoting the user mid-task even though the same model succeeds once native injection is disabled.

Changes

• packages/ai/src/api/anthropic-messages.ts: when the complete retryProviderRequest operation finally rejects, a numeric HTTP status carried by the SDK error is delivered once through options.onResponse before the error propagates; errors without a status (network, aborts) report nothing rather than a fabricated code.
• tool-search/service.ts: the provider-scope-scoped ToolSearchService carries a one-shot pending flag (noteNativeInjectionFailure/takeNativeInjectionFailure) recording that a native-injected request was rejected.
• tool-search/index.ts: the adapter's 400 onFallback records that reason on the service.
• agent-session.ts: the hard-error branch consumes the flag first — no tryFallback(), a zero-delay auto_retry_start, and the shared continuation on the SAME model (the adapter is already disabled for the session, and the retry rebuilds the request so the fix(ai): fold gateway-namespaced Anthropic tool references onto the request's tools #1480 reference repair applies). The flag is consumed once, so a second rejection takes the ordinary hard-error chain.

Evidence (all remote, mengmotaMac, bun 1.4.0)

• RED (test patch on 933307abb6): onResponse called 0 times on 400/500 rejections; takeNativeInjectionFailure is not a function; noteNativeInjectionFailure is not a function. The status-less case passes without fabrication.
• GREEN with the fix: new anthropic-on-response-error 3/3; test/tool-search/native-anthropic.test.ts 16/16; test/suite/retry-fallback-hard-error.test.ts 11/11 (2 new: one same-model retry with zero fallback events, one normal switch on the second 400); ai integrity + deferred-tools + provider-native-replay 52/52; retry-fallback-hardening + refusal + mcp/native-anthropic 20/20.
• bun run check green; changelog gate PASS (nearest trackers for both production paths + [Unreleased] entries).

Closes #1481, closes #1482.

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Summary by cubic

Fixes native tool-search 400 recovery so the same model is retried in place instead of demoting the user to a fallback, and delivers Anthropic HTTP error statuses that previously never reached the response hook.

• The Anthropic SDK rejects HTTP failures instead of returning a Response, so onResponse/after_provider_response never fired on error and the native adapter's 400 fallback was dead code.
• The rejection path now reports the SDK's numeric status once through onResponse before propagating; errors without a status (network, aborts) report nothing.
• After a native tool-search 400, the session retries once on the same model with zero delay since native injection is already disabled; only a second rejection takes the fallback chain.

Closes #1481 and #1482.

^{Written for commit 6bedcc1. Summary will update on new commits.}