Add initialize(InitializeRequest) overload to MCP client

[nquinquenel]

Summary

• Add initialize(InitializeRequest) overloads to McpAsyncClient and McpSyncClient, consistent with callTool(CallToolRequest)
• Thread caller-provided initialize requests through LifecycleInitializer, with client-side protocol version validation and defensive copying of _meta
• Remember a successful custom initialize request and resend it when the transport session is re-established (e.g. after McpTransportSessionNotFoundException); clear stored request on client close
• Document the overload, ordering constraint, and session-recovery behavior in docs/client.md

Design notes

This follows the approach agreed in #940 (not the closed builder-level initializeRequestMeta() approach from #1046).

Call initialize(InitializeRequest) before any other client method if custom metadata is required; lazy initialization triggered by methods like listTools() or callTool() uses the default request built from client builder settings.

Test plan

• ./mvnw -pl mcp-core -Dtest=LifecycleInitializerTests,McpAsyncClientTest,McpSyncClientTest test
• ./mvnw -pl mcp-core verify
• Unit tests cover custom request propagation, defensive _meta copy, unsupported protocol version rejection, init reuse, re-init with stored custom request, default request on re-init when none stored, and clear on close / closeGracefully
• Client wire tests cover async/sync paths via MockMcpClientTransport

Fixes #940