Fix headless-Chromium and temp-screenshot leaks on browser_preview error paths

Problem/Motivation

The browser_preview AI Agent tool (backed by scripts/browser-capture.mjs and src/PlaywrightRunner.php) leaks resources on error paths:

• scripts/browser-capture.mjs catches errors and calls fail(), which runs process.exit(0). This does NOT unwind the surrounding finally { browser.close() }, so the headless Chromium process is orphaned on every error path (navigation timeout, bad page, screenshot failure). Under load this leaks browser processes.
• PlaywrightRunner::capture() only unlinks the temporary screenshot PNG on the success path. A non-zero/timed-out Node run, or an ok=false JSON result, leaves the temp PNG in the temp directory forever — an unbounded temp-directory leak.

Steps to reproduce

1. Trigger the browser_preview AI Agent tool against a URL that times out or errors repeatedly.
2. Observe orphaned chromium/headless_shell processes accumulating (e.g. ps aux | grep headless_shell).
3. Observe /tmp/ai_playwright_*.png temp screenshot files accumulating and never being cleaned up.

Proposed resolution

• In scripts/browser-capture.mjs, close the browser in the catch block before calling fail(), so process.exit(0) never bypasses browser cleanup.
• In src/PlaywrightRunner.php, unlink the temp screenshot file on every exit path of capture() (success, non-zero/timed-out process, and ok=false JSON result), not only the success path.

Remaining tasks

• ✅ File an issue about this project
• ✅ Addition/Change/Update/Fix to this project
• ✅ Testing to ensure no regression
• ➖ Automated unit/functional testing coverage
• ➖ Developer Documentation support on feature change/addition
• ➖ User Guide Documentation support on feature change/addition
• ➖ UX/UI designer responsibilities
• ➖ Accessibility and Readability
• ❌ Reviewed by a human
• ❌ Code review by maintainers
• ❌ Full testing and approval
• ❌ Credit contributors
• ❌ Review with the product owner
• ❌ Update Release Notes
• ❌ Release

User interface changes

• N/A

API changes

• N/A

Data model changes

• N/A

Release notes snippet

• N/A