chat: preserve conversation ID for BYOK Responses

[Vritant Bhardwaj (vritant24)]

Summary

• Preserves the active chat conversation ID when BYOK requests cross the extension-contributed language model boundary.
• Lets Responses custom endpoints include the configured stable prompt_cache_key.
• Keeps prompt cache behavior unchanged when the setting is disabled or a request has no conversation ID.

Fixes #332031

Technical context for AI-assisted review

Intent and previous behavior

Agent requests provide a conversation ID, and the Responses serializer uses it to generate prompt_cache_key. Extension-contributed models rebuild the request on both sides of the vscode.lm boundary, but that bridge did not preserve the conversation ID. BYOK Responses requests therefore reached serialization without the value.

Implementation

The typed internal model-options contract now carries the conversation ID across the extension-contributed language model boundary. ExtensionContributedChatEndpoint encodes the value before invoking vscode.lm, and CopilotLanguageModelWrapper restores it when invoking the underlying BYOK endpoint.

Behavior and constraints

• The existing prompt-cache setting remains authoritative.
• Requests without a conversation ID still omit prompt_cache_key.
• The value is forwarded explicitly rather than derived from CapturingToken.chatSessionId, whose subagent semantics can differ from the originating conversation ID.
• Other request metadata omitted by this bridge remains outside this change.

[Copilot]

Pull request overview

Preserves conversation IDs across the extension language-model boundary so BYOK Responses endpoints can generate stable prompt cache keys.

Changes:

• Adds conversation ID to internal model options.
• Restores it in the BYOK endpoint request.
• Adds coverage for enabled, disabled, and missing-ID cases.

Show a summary per file

File	Description
extChatEndpoint.ts	Forwards conversation IDs through model options.
languageModelAccess.ts	Restores conversation IDs for endpoint requests.
customEndpointProvider.spec.ts	Tests prompt cache key behavior end to end.

Review details

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• Files reviewed: 3/3 changed files
• Comments generated: 0
• Review effort level: Balanced