editor: restore model-visible post-save diagnostics on the chat-diff save path #2210

Problem

PR #1403 (L1) makes the saveDirectly (chat-diff, PREVENT_FOCUS_DISRUPTION) path fire-and-forget its post-save diagnostics: DiffViewProvider.saveDirectly resolves immediately and the tail emits new problems only through the say("error") chat/webview channel. formatFileWriteResponse therefore no longer receives a newProblemsMessage on this path, so the model does not see the diagnostics that the diff-editor saveChanges path still reports.

This is intentional for the L1 latency goal (the save path no longer waits for the LSP settle), but it is a behavior difference between the two approval paths and should not be silent.

Proposed direction

• Restore a model-visible signal for new problems on the chat-diff path without blocking the save: e.g. persist the tail's problems into the next tool result / a queued non-interactive message the model sees, or return a promise-backed newProblemsMessage that the tool layer can await only when a problem is likely.
• Keep the say("error") UI row and its non-interactive semantics unchanged.
• Add tool-layer tests for WriteToFileTool/ApplyPatchTool/EditTool covering experiments: {} with a new problem: the model-visible response must include the problems text, while the save still resolves before the LSP settle.

Context

Found in the CodeRabbit review of #1403 (DiffViewProvider.saveDirectly); the current behavior is documented in that PR/issue. This issue tracks closing the gap between the diff-editor and chat-diff approval paths.