[Repo Assist] perf: use minimal TextEdit range to preserve cursor position during format

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🤖 This is an automated draft PR from Repo Assist, an AI assistant.

Summary

Replace the full-document TextEdit with a minimal-range TextEdit that covers only the characters actually changed by phpcbf.

Problem

The current formatter builds a TextEdit that spans the entire document:

let range = new Range(new Position(0, 0), lastLine.range.end);
resolve([new vscode.TextEdit(range, text)]);

VS Code applies this by replacing the complete document content. As a side-effect, the cursor jumps to position 0 on every format — even if the user was editing code in the middle or bottom of the file and phpcbf only fixed a few lines near the top.

This is a minor but annoying UX issue that affects every user who formats on save.

Fix

Add diffRegion(originalText, formattedText) to lib/utils.js. It finds the first and last character offset where the two strings differ, in O(n) time with no external dependencies. The unchanged prefix and suffix are excluded.

minimalEdits() in extension.js uses diffRegion + document.positionAt() to build a single TextEdit covering only that changed region:

// Before (full document replacement, cursor always jumps to 0):
resolve([new vscode.TextEdit(range, text)]);

// After (minimal replacement, cursor preserved outside the changed region):
resolve(minimalEdits(originalText, text, document));

Example: if phpcbf fixes indentation on lines 1–5 of a 200-line file, and the user's cursor is at line 150, the cursor no longer moves.

Trade-offs

Single TextEdit	Using one TextEdit keeps the implementation simple; a multi-edit Myers diff would give even smaller edits at higher complexity.
Line-ending sensitivity	positionAt() uses the document's own line endings; phpcbf output is written to a temp file and read back, so line endings should match the document.
Degenerate case	If the entire file changes, the region covers the whole document — same as before, no regression.

Test Status

✅ All 15 unit tests pass (node --test test/unit.test.js):

• 7 existing findFiles tests — unchanged
• 8 new diffRegion tests covering: identical strings, single-char changes, changes at start/end, line insertions, line deletions, and minimality of the unchanged suffix

✅ Syntax check: node --check extension.js — no errors

⚠️ Integration tests require a GUI environment and cannot run in CI.

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