fix: preserve Anthropic streaming overload errors in-band

[hustxiayang]

Description
Anthropic can return HTTP 200 for a streaming request and later report an overloaded_error in the Server-Sent Events (SSE) body. The current translator propagates that event as a Go error, so the client receives either a generic HTTP 500 before the response is committed or a terminated HTTP 200 stream after commitment. Neither result exposes the overload classification.

This change preserves HTTP 200 and translates an Anthropic streaming overload into a terminal Chat Completions SSE error:

data: {"error":{"type":"overloaded_error","message":"Overloaded"}}

The translator returns a typed AnthropicStreamError, and the response processor returns the serialized event as a normal body mutation only when its type is overloaded_error. The event is appended to any output already translated in the same callback. The processor then suppresses later provider chunks, omits [DONE], and records the request as failed. Other streaming error types and non-streaming errors retain their existing behavior.

Related issue

Fixes #2697

[netlify]

✅ Deploy Preview for theagentrouter ready!

Name	Link
🔨 Latest commit	464b56f
🔍 Latest deploy log	https://app.netlify.com/projects/theagentrouter/deploys/6ab6bd1ca5fb250008e6a13b
😎 Deploy Preview	https://deploy-preview-2698--theagentrouter.netlify.app
📱 Preview on mobile	Toggle QR Code... Use your smartphone camera to open QR code link.

---

To edit notification comments on pull requests, go to your Netlify project configuration.

[codecov]

Codecov Report

❌ Patch coverage is 90.74074% with 5 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
internal/extproc/processor_impl.go	90.32%	3 Missing ⚠️
internal/translator/anthropic_helper.go	91.30%	2 Missing ⚠️

📢 Thoughts on this report? Let us know!

[hustxiayang]

@yuzisun Could you please help to review it? Thanks a lot!