fix: map model_context_window_exceeded to length to prevent unknown loop

[heimoshuiyu]

Issue for this PR

Closes #18813
Related: #15778

Type of change

• Bug fix
• New feature
• Refactor / code improvement
• Documentation

What does this PR do?

Issue #18813 reports that some providers can return a non-standard finish reason:

• finish_reason: "model_context_window_exceeded"

From the report, z.ai returns this when context is too long; Anthropic appears to have introduced the same value (based on internet search, not directly verified via Anthropic API in this PR).

OpenCode session flow treats finish: "unknown" as unfinished. That can keep the loop running and repeatedly send requests.

This PR adds a guard in session processing for all models using @ai-sdk/openai-compatible:

• if a step finishes with finishReason === "unknown"
• and the step produced no text/reasoning/tool output

then OpenCode rewrites that finish reason to length.

This prevents the unknown-loop behavior described in #18813 and routes the result into normal length/context-limit handling.

(Additionally, the existing Copilot-specific mapper already handles model_context_window_exceeded directly.)

Reference implementation:

• fix(embedded): classify model_context_window_exceeded as context overflow, trigger compaction openclaw/openclaw#35934

How did you verify your code works?

• Rechecked the reported behavior and payload shape from z.ai/zhipuai finish_reason: "model_context_window_exceeded" is treated as unknown, causing loop instead of stopping #18813.
• Ran local tests:
  • bun test test/provider/copilot/copilot-chat-model.test.ts
  • bun test test/session/llm.test.ts

Screenshots / recordings

N/A (non-UI change). See screenshots and payload examples in #18813.

Checklist

• I have tested my changes locally
• I have not included unrelated changes in this PR

[github-actions]

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[heimoshuiyu]

oh no just realized this is a problem of z.ai or ai sdk, closing this fake pr sorry