Map OpenAI's context_length_exceeded errors (both the in-stream error
events sent by the Responses API / ChatGPT Codex backend and HTTP 400
responses carrying the code) to
LanguageModelCompletionError::PromptTooLarge. This stops pointless
retries and surfaces the dedicated token-limit UI instead of a raw error
string.

Also record a synthetic request token usage when a completion fails with
PromptTooLarge, so the context indicator reports Exceeded rather than
the stale usage of the last successful request, and the next prompt
qualifies for auto-compaction.

Ought to help a bit with
https://github.com/zed-industries/zed/issues/59600

Release Notes:

- N/A or Added/Fixed/Improved ...

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Co-authored-by: Claude Fable 5 <noreply@anthropic.com>
Co-authored-by: zed-zippy[bot] <234243425+zed-zippy[bot]@users.noreply.github.com>