fix(filesystem): improve edit_file error with nearest-match diagnostics

[Christian-Sidak]

Summary

When edit_file fails to find a match (neither exact nor whitespace-flexible), the error message currently just dumps the oldText with no context about what the file actually contains. This makes it hard for both agents and humans to diagnose why the match failed.

This PR improves the error to include:

• File path in the error message
• Closest near-match location with 1-indexed line numbers
• Match percentage (how many lines matched after trimming)
• Line-by-line comparison showing which lines matched and which diverged, with both the actual and expected content

Example error after this change:

Could not find match for edit in /path/to/file.ts
Closest match (75% of lines) at lines 42-45:
   line 42: "  const foo = bar;"
 ! line 43: "  baz();"
     expected: "  baz(x);"
   line 44: "  return result;"
   line 45: "}"

Edit oldText:
  const foo = bar;
  baz(x);
  return result;
}

No changes to the actual matching/replacement logic -- this is purely diagnostic improvement.

Fixes #2034

Test plan

• All 146 existing tests pass
• Updated test name and assertion to match new error format
• Verified error includes file path and diagnostic context on mismatch