fix: reply -32700 on stdio parse errors instead of closing

[DaleSeo]

Fixes #825

Motivation and Context

The RMCP stdio transport used to close the stream silently when it encountered a malformed line. This made it hard for clients to tell the difference between a parse failure and a normal end-of-file initiated by the peer. Just one bad byte could bring down the entire session.

Now, the stdio transport responds with a JSON-RPC -32700 Parse error when it receives a malformed line, instead of closing the stream silently. It also removes a leading UTF-8 BOM, so messages from Windows tools that have a BOM prefix can be parsed correctly.

How Has This Been Tested?

Added new tests

Breaking Changes

None.

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 Documentation
• My code follows the repository's style guidelines
• New and existing tests pass locally
• I have added appropriate error handling
• I have added or updated documentation as needed

Additional context