fix(filesystem): add guidance for failed edit matches

[Joosboy]

Description

Improves the filesystem server's edit_file failure message when an edit cannot be matched. The error now keeps the original failed oldText context and adds a short hint to inspect the current file contents with read_file before retrying.

Server Details

• Server: filesystem
• Changes to: edit_file error handling and unit tests

Motivation and Context

When oldText does not match the current file contents, the previous error only reported that no exact match was found. Issue #2034 describes this as difficult to recover from when the mismatch is caused by whitespace, indentation, or line-ending differences.

This keeps the edit behavior unchanged and makes the failure easier to act on.

Refs #2034.

How Has This Been Tested?

• npm test --workspace @modelcontextprotocol/server-filesystem -- __tests__/lib.test.ts
• npm run build --workspace @modelcontextprotocol/server-filesystem

Breaking Changes

No breaking changes.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation update

Checklist

• I have read the MCP Protocol Documentation
• My changes follows MCP security best practices
• I have updated the server's README accordingly
• I have tested this with an LLM client
• My code follows the repository's style guidelines
• New and existing tests pass locally
• I have added appropriate error handling
• I have documented all environment variables and configuration options

Additional context

The README and environment-variable checklist items are not applicable because this only changes an existing runtime error message and its test coverage.