refactor(vscode): harden context-usage display with trusted token limits

[yiliang114]

Summary

Follow-up to #2874. Adds defensive hardening so the context-usage bar behaves correctly even when the ACP server omits tokenLimit from its responses.

• knownTokenLimit() in core/tokenLimits.ts: a new export that returns undefined for unrecognized models (unlike tokenLimit() which always falls back to a default). This lets callers distinguish "known limit" from "guessed default".
• acpModelInfo.ts: derives _meta.contextLimit from the known-model table when the ACP payload omits a numeric limit, with a clear priority chain (legacy numeric > meta numeric > derived > explicit null > undefined).
• computeContextUsage(): extracted from the 45-line inline block in App.tsx into a standalone tested module. Returns null when no trusted numeric limit is available — the UI hides the context bar instead of showing a misleading default.
• Removes the last @qwen-code/qwen-code-core runtime import from App.tsx, keeping the webview bundle free of Node-only dependencies.

Files changed

File	Change
core/tokenLimits.ts	Add findTokenLimit() + knownTokenLimit() export, refactor tokenLimit() to reuse
acpModelInfo.ts	Derive contextLimit from known model table when ACP omits it
contextUsage.ts (new)	Pure function extracted from App.tsx, no default fallback
App.tsx	Replace 45-line inline block with computeContextUsage() call
+ 3 test files	Cover all new behaviors

Test plan

• tokenLimits.test.ts — knownTokenLimit returns values for known models, undefined for unknown
• acpModelInfo.test.ts — derives contextLimit when payload omits it, preserves explicit null
• contextUsage.test.ts — null without trusted limit, prefers usageStats over meta, inputTokens/promptTokens compat
• imageSupport.bundle.test.ts — App webview bundle has no core runtime imports

Depends on #2874

[github-actions]

📋 Review Summary

This PR adds defensive hardening for the context-usage display in the VS Code extension, ensuring correct behavior when the ACP server omits tokenLimit from responses. The implementation introduces a new knownTokenLimit() function that distinguishes between "known limit" and "guessed default," extracts context usage computation into a testable module, and removes the last runtime dependency on @qwen-code/qwen-code-core from the webview bundle. Overall, this is a well-structured refactoring with comprehensive test coverage.

🔍 General Feedback

• Strong separation of concerns: The extraction of computeContextUsage() from the 45-line inline block in App.tsx significantly improves testability and maintainability.
• Clear priority chain: The context limit resolution order (legacy numeric > meta numeric > derived > explicit null > undefined) is well-documented and logically sound.
• Excellent test coverage: All new functions have corresponding tests covering edge cases, including unknown models, null handling, and fallback behaviors.
• Clean dependency management: Removing the @qwen-code/qwen-code-core runtime import from App.tsx is a positive architectural improvement for the webview bundle.
• Consistent patterns: The refactoring of tokenLimit() to reuse knownTokenLimit() follows DRY principles without sacrificing clarity.

🎯 Specific Feedback

🔵 Low

• File: packages/vscode-ide-companion/src/utils/acpModelInfo.ts:66-73 - The priority chain comment is helpful, but consider extracting the context limit derivation logic into a named helper function (e.g., resolveContextLimit()) to improve readability and make the priority chain self-documenting:

  const contextLimit = resolveContextLimit(legacyLimit, metaLimit, derivedLimit);

• File: packages/vscode-ide-companion/src/webview/utils/contextUsage.ts:24 - The limit assignment could benefit from an explicit comment about why DEFAULT_TOKEN_LIMIT is intentionally NOT used as a fallback here (unlike in tokenLimit()), since this distinction is the core purpose of this PR.

• File: packages/core/src/core/tokenLimits.ts:190-200 - The findTokenLimit() function is declared but not exported. Consider adding a JSDoc comment explaining its internal-only status to prevent future accidental exports.

✅ Highlights

• packages/core/src/core/tokenLimits.ts - The refactoring of tokenLimit() to compose over knownTokenLimit() is elegant and maintains backward compatibility while enabling the new "unknown model" detection pattern.

• packages/vscode-ide-companion/src/webview/utils/contextUsage.ts - The pure function design with no default fallback is exactly what's needed for the webview context. Returning null when no trusted limit is available prevents misleading UI states.

• Test files - All three test files (tokenLimits.test.ts, acpModelInfo.test.ts, contextUsage.test.ts) demonstrate thorough coverage of edge cases, including the critical "unknown model returns undefined" behavior and the inputTokens/promptTokens compatibility handling.

• packages/vscode-ide-companion/src/utils/acpModelInfo.ts - The explicit handling of null vs undefined for context limits ("limit intentionally unknown" vs "not provided") shows thoughtful API design that respects server intent.