DRAFT - HttpRequest.Builder customizer

[Kehrlann]

⚠️ This a draft PR, to start a discussion. It is NOT a full implementation, has no tests, lacks docs, etc.

Provide a way to customize HTTP requests before executing them, for both HttpClientSseClientTransport and HttpClientStreamableHttpTransport.

We introduce AsyncHttpRequestCustomizer, which is the core class that does the heavy lifting. For blocking contexts, we provide a simpler SyncHttpRequestCustomizer API, which ultimately gets wrapped. The details of the implementation in both transports is not relevant (yet) ; we are focused on both customizers and also on the transpots, specifically:

• AsyncHttpRequestCustomizer
• SyncHttpRequestCustomizer
• HttpClientSseClientTransport.Builder#requestCustomizer
• HttpClientStreamableHttpTransport.Builder#requestCustomizer

Motivation and Context

These are transport-level hooks, to allow users to modify request, e.g. to add security in headers (OAuth2 tokens, API keys, etc).

How Has This Been Tested?

Tested with a Servlet Spring app, using SyncHttpRequestCustomizer.

Breaking Changes

No.

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