fix(grep): force --with-filename to fix single-file rg output parsing

[ShiraiEd]

Summary

• rg omits the filename prefix when searching a single file, causing the parser to misread the line number as the filename and truncate content at the
  first colon
• Added --with-filename to the rg invocation so output always includes the filename prefix regardless of how many files are searched
• Extracted inline parsing logic into parse_rg_line helper for testability

Test plan

• cargo fmt --all && cargo clippy --all-targets && cargo test
• Added test_parse_rg_line_with_colons_in_content — verifies filename and line number are correct when content contains colons
• Added test_parse_rg_line_single_file_fallback — verifies two-part lines fall back to the provided path
• Manual test: rtk grep "function" test.ts on a file with colon-heavy signatures now correctly returns 2 matches in 1 file with accurate filenames and line numbers

(fixes [#1613])

[CLAassistant]

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