Add McpTransportContext to McpSyncClient (#522)

* McpSyncClient: introduce McpTransportContext

- McpSyncClient should be considered thread-agnostic, and therefore
  consumers cannot rely on thread locals to propagate "context", e.g.
  pass down the Servlet request reference in a server context.
- This PR introduces a mechanism for populating an McpTransportContext
  before executing client operations, and reworks the HTTP request
  customizers to leverage that McpTransportContext.
- Move McpTransportContext from server to common package for shared client/server usage
- Make McpTransportContext immutable by removing put() and copy() methods
- Add static create() factory method for creating contexts with predefined data
- Update McpTransportContextExtractor to return context instead of modifying one
- Replace DefaultMcpTransportContext mutable implementation with immutable version
- Update all transport implementations to use McpTransportContext.EMPTY as default
- Rename *HttpRequestCustomizer -> *HttpClientRequestCustomizer
- Add end-to-end McpTransportContextIntegrationTests
- This PR introduces a breaking change to the Sync/Async request
  customizers.

Signed-off-by: Daniel Garnier-Moiroux <git@garnier.wf>