fix(edit-tool): eliminate token amplification and hang on large files

[xielixing]

Summary

Fixes #1650

The Edit tool caused extreme token amplification (957K input tokens) and a 1m23s hang on a 1185-line file. Two root causes:

Fix 1: Remove redundant dry-run from validate_input

validate_input() in file_edit_tool.rs ran a full dry-run apply_edit_to_content() — reading the file and generating all whitespace-normalization candidates — and then call_impl did the exact same work again when actually applying the edit. This doubled the file reads and candidate generation on every edit.

Fix: Removed the dry-run block from validate_input. The edit is already validated during call_impl via apply_edit_to_content, so the dry-run was purely redundant work.

Fix 2: Add fast path before candidate generation in apply_edit_to_content

edit_string_candidates() generates whitespace-normalization candidates (tabs↔spaces at width 2 and 4), each calling find_actual_string() which does O(n*m) char-by-char scanning. ALL candidates were generated upfront before any matching, even when the exact old_string already matched the file content.

Fix: Added a fast path that tries the exact old_string/new_string match via apply_match_and_replace() before calling edit_string_candidates(). If the exact match succeeds (the common case), it returns immediately — skipping all candidate generation and expensive scanning. If the exact match fails with "not found", it falls through to the existing candidate loop (slow path unchanged for edge cases).

Validation

• cargo check -p tool-runtime -p bitfun-core — passed
• cargo test -p tool-runtime -- fs::edit_file — 21/21 passed
• cargo test -p bitfun-core -- file_edit_tool — 3/3 passed

Impact

For the reported 1185-line file edit, the common case (exact match) now completes in a single apply_match_and_replace call instead of generating and scanning multiple whitespace-normalization candidates through find_actual_string. Combined with removing the redundant dry-run, this eliminates the token amplification and hang.