Hermetic webview HTTP test fixture migration

[DrumRobot]

Summary

packages/vscode-extension/src/test/suite/webview.test.ts:379-462 (the session-page HTTP test) essentially never runs in automation:

• CI: skipped via process.env.CI check
• Local without ~/.claude/projects/ populated: skipped via findFirstSession() === null
• Local with ~/.claude/projects/ populated: runs, but depends on whichever session happens to be first on disk — non-deterministic

A test that essentially never runs in automation provides regression detection only when a developer happens to run it locally. The risk it's supposed to prevent (a runtime bug in /session/{project}/{id} SSR) goes uncaught.

Origin

Raised by CodeRabbit on PR #163, confirmed valid by Internal Code Review (partial agreement — see PR #163 review comment). Deliberately deferred out of PR #163's scope to avoid a 4-test refactor blast radius.

Proposed fix

CodeRabbit's suggestion: set CLAUDE_SESSIONS_DIR to a fixture directory (e.g. packages/test-fixtures/sessions) in runTest.ts, and update the test to seed a deterministic fixture session so findFirstSession() always finds it, regardless of CI or local environment.

Scope caution: this is not a 1-test change. Changing the CLAUDE_SESSIONS_DIR data source affects:

• The 3 sibling HTTP tests in the same suite (Web server responds with HTTP 200, Frontend root path responds with HTTP 200, Open Web UI opens external browser)
• path.test.ts:117-123, which already branches on this env var

A coordinated migration of all 4 HTTP tests to the fixture-based approach is needed, not an isolated fix to the session-page test alone.

Acceptance criteria

• All 4 HTTP tests in webview.test.ts (and any others sharing the CI-skip/real-data pattern) use a deterministic fixture session directory
• Tests run in CI (no process.env.CI skip needed once data is deterministic)
• path.test.ts:117-123's existing branch on CLAUDE_SESSIONS_DIR is verified compatible with the new fixture setup