Implement DNS Rebinding Protections per spec

[ddworken]

Motivation and Context

This implements the mitigations described here. To avoid breaking existing applications this doesn't enable any changes by-default, but enabling this feature is heavily encouraged for any local MCP servers using the SSE transport.

How Has This Been Tested?

Tested via unit tests.

Breaking Changes

To avoid introducing any breaking changes, the DNS rebinding protections are disabled by default. Ideally we should find a way to enable them by default. But for now, adding them as disabled is a good first step.

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

[jerome3o-anthropic]

lgtm - thanks for this

[ihrpr]

Thank you @ddworken! Please can you merge the main and I'll merge?

[ddworken]

Thanks! I fixed the merge conflict and confirmed all unit tests are passing locally.