JDK HTTP Client Streamable HTTP support

[taobaorun]

Motivation and Context

JDK HTTP Client-based client transport for Streamable HTTP. Reference the WebClient Streamable HTTP implementation. #292

How Has This Been Tested?

• Regular integration tests using the everything MCP server.

Breaking Changes

The client initialization became lazy and more error-prone - connections happen upon first request and initialization is performed implicitly if it has not been issued before.

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

Related #310

[noear]

good job