feat(client): add WebFluxStreamableClientTransport.java

[Yeaury]

add WebFluxStreamableClientTransport.java

Motivation and Context

Add client support for 2025-03-26 protocol version

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](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

[edwardcapriolo]

All just nitpicky comments.

[edwardcapriolo]

Just from an outsider it is hard to understand what all this does without a test of some sort. Maybe at least some javaadoc.

[Yeaury]

First of all thank you for your guidance

This PR primarily aims to support streaming HTTP transport for the 2025-03-26 protocol version.

The main changes are as follows:
The client sends an initialization POST request to the /message endpoint, and the server may optionally generate and return a session ID to the client.
All messages are sent via HTTP POST requests to the /message endpoint. If a session ID exists, it MUST be included in the request.
The client handles responses from the server accordingly based on the protocol specifications.

I have verified the changes locally, and test cases or Javadoc will be added in subsequent updates.

[chickenlj]

It's better to add a UT for the code, demonstrating both how it works and it can work as expected.