fix: stop spinner on failed file edits and add re-anchor experiment (#12032)

[roomote-v0]

Summary

This PR attempts to address Issue #12032. Feedback and guidance are welcome.

Part 1: Fix spinner stuck on failed file edits (UI bug)

Problem: When a file edit tool (e.g., apply_diff) fails due to LLM hallucination, the loading spinner at the top of the UI keeps spinning indefinitely. This happens because handlePartial() creates a partial ask message during streaming, but when the edit fails in execute(), no code finalizes (closes) that partial ask message.

Fix:

• ApplyDiffTool: Added finalizePartialToolAskIfNeeded() helper (matching the pattern already in EditFileTool) that closes any outstanding partial ask message on error paths. Added didSendPartialToolAsk and partialToolAskRelPath tracking properties, set in handlePartial().
• ApplyDiffTool: Always show diff_error message on every failure (previously only shown on 2nd consecutive failure), giving immediate visual feedback.
• ApplyDiffTool: Set didToolFailInCurrentTurn = true on diff apply failure for consistency.
• EditFileTool: Always show diff_error message on every failure (previously only shown on 2nd consecutive failure).

Part 2: Re-anchor before edit experiment

Problem: LLMs hallucinate file contents when editing, especially for longer files or after many conversation turns. The issue author reports this can be architecturally solved in 80%+ of cases by requiring a re-read step before every edit.

Solution: Added a new experimental setting reAnchorBeforeEdit that, when enabled, adds a system prompt guideline instructing the model to always re-read a file (using read_file) immediately before editing it. This uses more tokens but significantly reduces hallucination-based edit failures.

Changes:

• packages/types/src/experiment.ts: Added reAnchorBeforeEdit to experiment IDs and schema
• src/shared/experiments.ts: Added RE_ANCHOR_BEFORE_EDIT config (default: off)
• src/core/prompts/sections/tool-use-guidelines.ts: Accepts experiments config, adds re-anchoring instruction when enabled
• src/core/prompts/system.ts: Passes experiments to getToolUseGuidelinesSection()
• webview-ui/src/i18n/locales/en/settings.json: Added localization keys
• Settings UI auto-renders via existing ExperimentalSettings component

Tests

• Updated editFileTool.spec.ts to reflect that diff_error is now shown on every failure
• Updated experiments.spec.ts with new experiment tests
• Added tool-use-guidelines.spec.ts to verify re-anchor guideline behavior

All existing and new tests pass. Lint and type checks pass.

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