fix: handle Windows backslash path separator in grep_search tool

[rekha0suthar]

Problem

On Windows, ripgrep outputs file paths prefixed with a backslash separator (e.g. .\path\to\file.ts) instead of the Unix-style forward slash (e.g. ./path/to/file.ts).

The formatGrepSearchResults function in core/util/grepSearch.ts only checked for line.startsWith("./") when identifying file header lines, so on Windows none of the lines were detected as file paths, and the grep_search tool silently returned zero results for every query.

Root Cause

// Before — only matches Unix paths
if (line.startsWith("./") || line === "--") {

Fix

Added a second check for the Windows backslash prefix:

// After — matches both Unix and Windows paths
if (line.startsWith("./") || line.startsWith(".\\") || line === "--") {

This is a one-line surgical fix with no risk of affecting Unix behaviour.

Testing

• On Unix: existing behaviour preserved — ./ prefix still matched correctly
• On Windows: .\ prefix now detected, file headers correctly identified, results returned

Fixes #13027

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