chore(http): create constant for session id header

[EdouardCourty]

Motivation and Context

The Mcp-Session-Id value is re-written a few times in the codebase. It should be stored in a constant to avoid duplication.

How Has This Been Tested?

Tests are passing.

Breaking Changes

No behavioral or API change.

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
• Refactor

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

[chr-hertel]

let's not reuse the const in tests, and only go with private scope - maybe shorter as well?

Suggested change

	public const MCP_SESSION_ID_HEADER = 'Mcp-Session-Id';
	private const SESSION_HEADER = 'Mcp-Session-Id';

[chr-hertel]

what about introducing a const here as well?

    private const ALLOWED_HEADER = [
        'Accept',
        'Authorization',
        'Content-Type',
        'Last-Event-ID',
        'Mcp-Protocol-Version',
        self::SESSION_HEADER,
    ];

in combination with:

Suggested change

	'Access-Control-Allow-Headers' => 'Content-Type, '.self::MCP_SESSION_ID_HEADER.', Mcp-Protocol-Version, Last-Event-ID, Authorization, Accept',
	'Access-Control-Allow-Headers' => implode(',', self::ALLOWED_HEADER),

[chr-hertel]

Thanks @EdouardCourty - small adjustments. In general I'm not in favor of having too much coupling of code between tests and implementation - rather a nit pick, i know 👼