fix(mcp-filesystem): preserve literal edit replacement text - #1979

Summary

Fixes #1976

edit_file passed newText directly to JavaScript's String.replace, so $&, $$, $', and `$`` were interpreted as replacement patterns instead of being written literally. This could silently preserve the old text, remove dollar signs, or duplicate surrounding content.

The exact-match path now uses a replacement callback, which returns the requested text verbatim. The existing line-by-line fallback is unchanged.

Verification

• Added MCP in-memory regression coverage for ordinary text, $&, $$, $', and `$``.
• Covered sequential edits and dryRun behavior.
• pnpm --filter @agent-infra/mcp-server-filesystem test — 5 passed.
• pnpm exec prettier --check and pnpm exec eslint on changed files — passed.
• pnpm exec tsc -p packages/agent-infra/mcp-servers/filesystem/tsconfig.json --noEmit — passed.
• Built @agent-infra/logger, mcp-http-server, and @agent-infra/mcp-server-filesystem in dependency order; ESM, CJS, and declaration output succeeded.
• git diff --check — passed.

Checklist

• Added or updated necessary tests.
• Updated documentation to align with changes (not applicable; no user-facing API text changed).
• Verified no breaking changes.
• My change does not involve the above items.

AI assistance

Codex assisted with issue investigation and test design. The submitted implementation and every changed line were reviewed against the issue, repository conventions, and the validation results above.