fix(core): recognise Windows backslash paths in grep_search output

[guillaume-flambard]

Problem

Fixes #13027.

grep_search always returns "The search returned no results" on Windows, even when ripgrep finds matches (verified by the reporter with Process Monitor: rg.exe runs and returns results, but the tool reports none).

Root cause

formatGrepSearchResults in core/util/grepSearch.ts detects ripgrep file-heading lines with:

if (line.startsWith("./") || line === "--") {

ripgrep emits headings as the relative path — ./dir/file on POSIX, but .\dir\file on Windows. The startsWith("./") check never matches the Windows form, so no line is counted as a heading, numResults stays 0, and every match is silently discarded. Diagnosed precisely in the issue by @SpikedCola.

Fix

Also accept the .\ prefix:

if (line.startsWith("./") || line.startsWith(".\\") || line === "--") {

POSIX ./ headings and the -- context separator are unchanged.

Tests

Added a unit test in core/util/grepSearch.vitest.ts that feeds backslash-separated ripgrep output and asserts numResults > 0 and the filenames are present — it fails before the change (0 results) and passes after.

I verified the header-detection logic in isolation (the Windows heading goes from unrecognised → recognised, POSIX and -- still recognised). I don't have a Windows machine to run the full agent end-to-end, but the parser is pure and the added test covers the regression in CI.

[github-actions]

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[SpikedCola]

I have read the CLA Document and I hereby sign the CLA

[guillaume-flambard]

I have read the CLA Document and I hereby sign the CLA

[guillaume-flambard]

Heads-up on CI: jetbrains-tests is red here, but it's red on every other open PR right now too (#13036, #13034, #13033, #13032, #13031) — a pre-existing failure unrelated to this change. This PR only touches core/util/grepSearch.ts (pure TS parsing); core-checks, which runs the grep unit tests including the new backslash-path case, is green. The build-and-upload-vsix (darwin) cancel and require-all-checks-to-pass failure are both cascades from that jetbrains job.