fix(filesystem): show context for failed edits

[go165]

Fixes #2034.

Summary:

• expand edit_file not-found errors with line-numbered file context
• include a re-read hint so clients can copy exact current text before retrying
• add a regression test for the diagnostic message

Validation:

• npm exec --workspace src/filesystem -- vitest run tests/lib.test.ts --coverage=false: 47 passed
• git diff --check: passed
• npm run --workspace src/filesystem build: blocked locally because this separate worktree cannot resolve @modelcontextprotocol/sdk imports from the temporary linked node_modules setup