fix(core): treat relatedRequestId 0 as present in debounce guard

[utkarshsharma19]

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 (https://modelcontextprotocol.io)
• 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

This is intentionally narrow: global simple notifications without a
relatedRequestId still debounce as before.

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commit: cfa5528