fix(vscode): restore live context usage on v2

[LCZcn96]

Related Issue

Fixes #3059

Problem

After the VS Code extension switched to the v2 engine, the context-window usage indicator could disappear or remain frozen. v2 status updates carried contextTokens and maxContextTokens, but the legacy VS Code Webview contract was not receiving a derived context_usage value. Re-entering a session also dropped the value because announceStatus() did not forward the status snapshot's context usage.

What changed

• Derive context_usage in the VS Code event adapter when v2 status events omit the ratio but provide a valid token pair.
• Restore contextUsage in the v2 Node SDK status snapshot so downstream consumers receive the combined status contract.
• Forward context_usage when a session announces its current status to an attaching Webview.
• Preserve explicit ratios and omit derived ratios when the context capacity is invalid.
• Add regression tests for live v2 status mapping, invalid capacities, SDK snapshots, and attach status.

Verification

• I have read the CONTRIBUTING document.
• I have linked a related issue.
• I have added tests that prove the fix works.
• Ran the gen-changesets skill; added a patch changeset for @moonshot-ai/kimi-code.
• No documentation update is needed.

Tests run with Node v24.15.0 and pnpm 10.33.0:

• VS Code targeted tests: 4 files, 114 passed
• Node SDK session wiring tests: 3 passed
• VS Code typecheck: passed
• Node SDK typecheck: passed

[changeset-bot]

🦋 Changeset detected

Latest commit: f4fbbb9

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

Name	Type
@moonshot-ai/kimi-code	Patch

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