fix(vscode-ide-companion): preserve explicit null context limits

[yiliang114]

Summary

• respect explicit contextLimit: null values from ACP instead of overriding them with a derived known-model limit
• keep the derived-limit fallback only for payloads that omit contextLimit
• remove the now-unused browser-side utils/tokenLimits.ts helper after the context-usage refactor in fix(vscode): force fresh ACP session on new-session action #2874

Testing

• cd packages/vscode-ide-companion && npm run lint
• cd packages/vscode-ide-companion && npm run check-types
• cd packages/vscode-ide-companion && npx vitest run src/utils/acpModelInfo.test.ts src/utils/imageSupport.bundle.test.ts
• cd packages/vscode-ide-companion && npm run build:dev

Context

• Follow-up to fix(vscode): force fresh ACP session on new-session action #2874
• This PR is stacked on fix/2847-vscode-new-session-minimal so the original bugfix PR can stay focused while the review polish lands separately.

[github-actions]

📋 Review Summary

This PR is a well-crafted follow-up to #2874 that fixes how explicit contextLimit: null values from ACP are handled. The changes ensure that explicit nulls from the server are preserved rather than being overridden by derived known-model limits, while maintaining the derived-limit fallback for payloads that omit contextLimit. The removal of the now-unused tokenLimits.ts helper is appropriate given the context-usage refactor.

🔍 General Feedback

• Clean, focused changes: The PR makes minimal, surgical changes to address the specific issue without introducing unnecessary complexity.
• Good test coverage: New test cases cover both extractSessionModelState and extractModelInfoFromNewSessionResult for the explicit null preservation behavior.
• Clear priority reordering: The comment update accurately reflects the new priority order for context limit resolution.
• Appropriate cleanup: Removing tokenLimits.ts after the refactor in fix(vscode): force fresh ACP session on new-session action #2874 prevents dead code accumulation.

🎯 Specific Feedback

🟢 Medium

• File: packages/vscode-ide-companion/src/utils/acpModelInfo.ts:68-74 - The priority comment is helpful, but consider adding a brief example or reference to the ACP spec that documents what contextLimit: null semantically means ("limit intentionally unknown"). This would help future maintainers understand why explicit null takes precedence over derived limits.

🔵 Low

• File: packages/vscode-ide-companion/src/utils/acpModelInfo.test.ts:255 - The renamed test "derives contextLimit for known models when metadata omits contextLimit" is clearer than the original, but consider also testing the edge case where _meta is explicitly null vs {} to ensure both paths are covered (the current test uses {}).

• File: packages/vscode-ide-companion/src/utils/acpModelInfo.ts - After removing the knownTokenLimit import and getContextLimitFromMeta helper, consider whether any inline documentation should be added to explain the contextLimit resolution logic for readers unfamiliar with the ACP protocol semantics.

✅ Highlights

• Excellent test additions: The two new test cases ('preserves explicit null contextLimit for known models') clearly demonstrate the intended behavior and protect against regressions.
• Thoughtful prioritization: The reordering of the priority chain correctly respects server intent while maintaining sensible fallbacks.
• Good cleanup discipline: Removing tokenLimits.ts (196 lines) demonstrates good hygiene in eliminating dead code after refactors.

[tanzhenxin]

Review

Clean follow-up to #2874. Deletes the duplicated 196-line tokenLimits.ts, preserves explicit null context limits from ACP, and consolidates state-clearing into resetConversationState. Good test coverage.

Verdict

APPROVE