fix(vscode): fix pipeline test and lint failures

[marius-kilocode]

What Problem This Solves

The VS Code pipeline failed because a Happy DOM fixture did not expose IntersectionObserver, and the session context exceeded the enforced ESLint file-size limit.

Why This Change Was Made

The activity fixture now exposes the browser API used by the mounted UI. The session context value type is moved to a dedicated type module, and the source contract test follows the type declaration.

User Impact

No production behavior changes. The VS Code unit-test and production build checks can complete successfully.

Evidence

• Full VS Code unit suite: 4,405 passed
• Production build check: passed
• ESLint: passed
• Webview typecheck: passed
• Prettier, Knip, and marker checks: passed

[kilo-code-bot]

Code Review Summary

Status: No Issues Found | Recommendation: Merge

Files Reviewed (4 files)

• packages/kilo-vscode/tests/fixtures/session-provider-activity.tsx
• packages/kilo-vscode/tests/unit/prompt-send-contract.test.ts
• packages/kilo-vscode/webview-ui/src/context/session-types.ts
• packages/kilo-vscode/webview-ui/src/context/session.tsx

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_{Reviewed by grok-4.6 · Input: 60K · Output: 6.5K · Cached: 308.5K}

_{Review guidance: REVIEW.md from base branch main}