Properly check content type. Should fix #450

[nerzhulart]

Motivation and Context

How Has This Been Tested?

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 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

[Copilot]

Pull request overview

This PR fixes issue #450 by correcting the content type validation in the SSE server transport. The change updates the equality check to use the proper content type matching method, which handles content type parameters correctly (such as charset specifications).

Key Changes:

• Updated content type validation to use match() instead of direct equality comparison

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