Tracking L1: async post-save diagnostics on chat-diff save path

[easonLiangWorldedtech]

Tracking issue for the L1 PR (branch feat/async-save-diagnostics-l1; PR to be opened after review).

Part of the file-write safety series — plan: easonLiangWorldedtech/Zoo-Code#33, epic: #1375. Item: L1 — Async post-save diagnostics. Stacked on #1395 (S3 atomic publish); to be rebased onto main once #1395 lands.

Scope

• DiffViewProvider.saveDirectly no longer blocks the save result on LSP diagnostics: the write and response resolve immediately, and post-save diagnostics become an asynchronous follow-up once settled (the existing writeDelayMs moves into that tail).
• The follow-up is emitted through the existing ClineSay type error, with text of the form New problems detected after saving file: <relPath> — no new say type, and packages/types stays untouched.
• diagnosticsEnabled = false (the current saveDirectly parameter) means no diagnostics at all, exactly as today.
• Because diagnostics no longer run before the response, the tool-result JSON drops the problems field — the information is delivered later via the follow-up event instead.

Acceptance

• Save resolves without waiting on diagnostics (post-save latency drops by the LSP-settle time); diagnostic information is still delivered, just later
• Follow-up event fires with the settled diagnostic payload when new problems exist; no event on a clean save
• No floating promises — the follow-up tail is explicitly handled
• CI green (ubuntu unit-test gate), 100% codecov patch coverage

Status

• PR: #1403 (head 13b60328c; ALL checks green incl. ubuntu+windows unit, e2e-mock, codecov/patch; CodeRabbit review complete; awaiting maintainer merge)