docs: add GET request handling for streamableHttp stateless mode

[saharis9988]

see #1150 for reference

Motivation and Context

This PR adds an example demonstrating how to properly return 405 for the /mcp GET endpoint in stateless mode, ensuring correct client behavior and aligning the documentation with the MCP specification.

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

[pkg-pr-new]

Open in StackBlitz

npm i https://pkg.pr.new/modelcontextprotocol/typescript-sdk/@modelcontextprotocol/sdk@1161

commit: 19cc793