Don't crash BYOK/custom models when the response hits the length limit

[vritant24]

Fixes #319592

When a custom/BYOK model returns finish_reason: "length" (hit its context
window), we threw Error: Response too long. and discarded everything the model
already streamed.

By the time we get the Length result the partial text has already been
streamed via the finished callback, so instead of throwing we just resolve and
keep it. This matches how the rest of the codebase already handles Length
(defaultIntentRequestHandler, agentIntent, codeMapper all use the partial
text rather than failing).

[Copilot]

Pull request overview

This PR fixes issue #319592, where custom/BYOK OpenAI-compatible endpoints that return finish_reason: "length" (i.e. hit their context window) caused the Copilot language-model wrapper to throw Error: Response too long. and discard all text the model had already streamed. The fix intercepts the ChatFetchResponseType.Length result in CopilotLanguageModelWrapper._provideLanguageModelResponse and resolves gracefully (returning undefined) instead of throwing, since the partial text was already reported to the consumer via the streaming finishedCb. A warning is logged, and a unit test verifies the truncated response no longer surfaces as a hard failure.

Changes:

• Treat a Length finish reason as a successful (truncated) response rather than throwing, keeping the already-streamed partial text.
• Log a warning when a response is truncated due to the length limit.
• Add a unit test asserting provideLanguageModelResponse does not throw on a Length response.

Show a summary per file

File	Description
extensions/copilot/src/extension/conversation/vscode-node/languageModelAccess.ts	Adds an early return for ChatFetchResponseType.Length before the failure path, logging a warning and returning undefined instead of throwing "Response too long."
extensions/copilot/src/extension/conversation/vscode-node/test/languageModelAccess.test.ts	Adds a "length finish reason" suite verifying that a truncated (Length) response does not throw.

Review details

• Files reviewed: 2/2 changed files
• Comments generated: 0
• Review effort level: Medium