Continue chat-session cleanup when one session fails during site delete

[bcotrim]

Related issues

• Related to Delete AI chat sessions when a site is deleted #4132
• Addresses @sejas's review suggestion from that PR

How AI was used in this PR

AI was used to implement the focused robustness change, add the regression test, and run validation.

Proposed Changes

• Ensure deleting a site continues removing its remaining AI chat sessions when one session's metadata cleanup fails.
• Preserve best-effort metadata cleanup when a session file is already missing.

Testing Instructions

• Run npx eslint --fix packages/common/ai/sessions/manage.ts packages/common/ai/sessions/tests/delete-sessions-for-site.test.ts.
• Run npm run typecheck.
• Run npm test -- packages/common/ai/sessions/tests/delete-sessions-for-site.test.ts and confirm all three tests pass.

Pre-merge Checklist

• Have you checked for TypeScript, React or other console errors?

[sejas]

Thanks! Code looks good!

[wpmobilebot]

📊 Performance Test Results

Comparing b925f72 vs trunk

app-size

Metric	trunk	b925f72	Diff	Change
App Size (Mac)	1361.90 MB	1361.90 MB	+0.00 MB	⚪ 0.0%

site-editor

Metric	trunk	b925f72	Diff	Change
load	1084 ms	1087 ms	+3 ms	⚪ 0.0%

site-startup

Metric	trunk	b925f72	Diff	Change
siteCreation	7001 ms	7013 ms	+12 ms	⚪ 0.0%
siteStartup	2360 ms	2356 ms	4 ms	⚪ 0.0%

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Results are median values from multiple test runs.

Legend: 🟢 Improvement (faster) | 🔴 Regression (slower) | ⚪ No change (<50ms diff)