Honor store: false when recording Responses API stateful markers in BYOK

[Davixk]

Fixes #325436

Description

BYOK custom endpoints using the Responses API record a stateful marker from every response.completed, even when the server answered store: false. The next request is then truncated to the items after the marker while a later guard strips the non-resp_ previous_response_id, producing an unanswerable request (a tool_result without its tool_use, or no user message at all).

Two changes:

• OpenAIResponsesProcessor emits the stateful marker only when response.store !== false, so a server that declares no persistence never gets a marker recorded and later requests carry the full input. Responses without a store field keep today's behavior, so CAPI is unaffected.
• The BYOK Responses branch validates the pending marker before the body is built: a marker not shaped resp_... sets ignoreStatefulMarker, keeping truncation and previous_response_id a single decision. The late guard remains as an invariant. This also heals existing transcripts that already carry foreign markers.

The previous_response_not_found retry flow is untouched and remains the fallback for expired server state.

How to test

1. Configure a customendpoint provider with "apiType": "responses" against a server that echoes store: false.
2. Run a tool-calling turn, then a follow-up in the same conversation.
3. Before: the follow-up is truncated to post-marker items with no previous_response_id and is rejected. After: the follow-up carries the full input and succeeds.

Added spec coverage: responsesApi.spec.ts (marker emitted for store: true and absent, not for store: false) and openAIEndpoint.spec.ts (a non-resp_ marker in history yields full input and no previous_response_id).

[Copilot]

Pull request overview

Prevents stateless BYOK Responses endpoints from producing truncated follow-up requests.

Changes:

• Suppresses stateful markers when store: false.
• Rejects non-resp_ markers before request conversion.
• Adds regression coverage for both behaviors.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated no comments.

File	Description
extensions/copilot/src/platform/endpoint/node/responsesApi.ts	Honors the completed response’s store flag.
extensions/copilot/src/platform/endpoint/node/test/responsesApi.spec.ts	Tests marker emission across store values.
extensions/copilot/src/extension/byok/node/openAIEndpoint.ts	Validates markers before input truncation.
extensions/copilot/src/extension/byok/node/test/openAIEndpoint.spec.ts	Verifies foreign markers preserve full input.