agentHost: preserve BYOK state through tool continuations

[sgent-epic]

Summary

• retain provider response IDs only while tool calls are pending
• recover the Copilot SDK immediate tool-result replay by supplying previous_response_id and forwarding only the matching tool outputs
• scope pending state by authenticated SDK session, vendor, and model
• preserve explicit continuation IDs and clear recovered state after a terminal response

Why

The bundled Copilot SDK currently replays the full Responses history after a tool call and omits previous_response_id. Extension-provided stateful models then receive no stateful_marker and reject the continuation.

The integration test reproduces the SDK request shape directly. Before the proxy change, it failed because the second request replayed the message, reasoning, and tool call with no response ID. It now reaches the provider with the original response ID and only the tool output.

Fixes #329283

Testing

• npm run compile-client
• scripts\test.bat --run src\vs\platform\agentHost\test\node\byokLmProxyService.test.ts
• scripts\test-integration.bat --run src\vs\platform\agentHost\test\node\providerIntegration\copilotByokResponses.integrationTest.ts
• precommit hygiene
• git diff --check upstream/main...HEAD

[Copilot]

Pull request overview

Preserves BYOK provider state through Copilot SDK tool continuations, addressing #329283.

Changes:

• Tracks pending response IDs by session, vendor, and model.
• Recovers matching tool-output continuations without overriding explicit state.
• Adds unit and SDK integration coverage.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 1 comment.

File	Description
byokLmProxyService.ts	Implements continuation recovery and scoped state tracking.
byokLmProxyService.test.ts	Tests recovery, scoping, explicit IDs, and cleanup.
copilotByokResponses.integrationTest.ts	Verifies SDK tool continuation behavior end-to-end.

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[sgent-epic]

Fixed in e547f44 with a 256-entry recency cap and oldest-entry eviction. Regression coverage verifies resumable continuations are preserved.

[sgent-epic]

Hi Vritant Bhardwaj (@vritant24), I hope you're doing well. Is there any way this could be prioritized? It's a blocker for wider piloting of agents in VS Code at Epic. I'm personally really liking the VS Code environment over other options, and I'd like to enable other folks to give it a try as well. Thanks for your time!