DRAFT: Add McpTransportContext in clients

[Kehrlann]

This PR adds McpTransportContext to MCP Clients, and makes it available to (Async|Sync)HttpRequestCustomizers.

Motivation and Context

The motivation is twofold:

• It creates a unified API for reading the "context" in which an MCP Client request is issued, regardless of sync/async. It can be consumed by HttpClient-based transports through HttpRequestCustomizer, or through the reactor context in WebClient-based transports.
• It also addresses limitations with thread local usage in request customizers, see TransportContext for Client side #479

How Has This Been Tested?

TODO

Breaking Changes

n/a

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