feat: Implement Stateless MCP (SEP-2575) support

[stenalpjolly]

Summary

Implement Stateless MCP (SEP-2575) support in the Java SDK. This includes bypassing connection handshake initialization for version 2026-06-18, embedding namespace-prefixed protocol metadata inside _meta on all outgoing tool requests, setting the MCP-Protocol-Version HTTP request header, and implementing dynamic client-side fallback/retry version negotiation when target servers return version mismatch errors (-32004 or -32001).

Expectation & Implementation

• New Protocol Version: Registered stateless protocol version 2026-06-18 in ProtocolVersion.
• JSON-RPC Model Extension: Added _meta map to CallToolParams to support inline protocol metadata.
• Stateless HTTP Transport Subclass: Created HttpMcpTransportV20260618 which inherits from BaseMcpTransport. It overrides modifyRequestParams to inject reverse-DNS prefixed _meta keys (io.modelcontextprotocol/protocolVersion, etc.), sets the MCP-Protocol-Version header, and overrides ensureInitialized to bypass stateful handshake checks.
• Base Transport Hook: Added modifyRequestParams request lifecycle hook in BaseMcpTransport.
• Dynamic Version Switching: Implemented switchVersion(ProtocolVersion) in HttpMcpTransport to reconstruct delegate transports.
• Exception Mapping: Caught JSON-RPC error codes -32004 (Unsupported protocol version) and -32001 (Method not found) during response deserialization, parsed server-supported versions, and threw a custom McpProtocolNegotiationException.
• Client Fallback Retry: Wrapped client requests in executeWithFallback in McpToolboxClientImpl which catches McpProtocolNegotiationException, selects the highest mutually supported version, invokes switchVersion on the transport, and retries the request exactly once.

Test cases

• Created HttpMcpTransportStatelessTest to verify:
  • Bypassing initialize handshake for 2026-06-18 transport.
  • Embedding correct metadata in request _meta and MCP-Protocol-Version header.
  • Throwing McpProtocolNegotiationException with compatible version lists.
  • Client-level automatic retry and version switching fallback.
• Enhanced HttpMcpTransportTest to cover all error states (non-200, missing session ID, version mismatches) across all stateful transport versions.
• Added comprehensive enum and close check coverage.

Acceptance criteria

• All compilation, unit tests, formatting (using google-java-format), checkstyle, and Jacoco coverage checks (>98%) pass.

Breaking changes

• No public API breaking changes. The client wrapper handles version fallbacks transparently.