fix: release transport-owned HttpClient references on close

[TsengX]

Summary

Fixes the Java 17 HttpClient-* SelectorManager thread retention reported in #547 and #620 by releasing transport-owned HttpClient references on close instead of forcing internal client shutdown.

This change:

• preserves the current public builder API
• keeps Java 17 compatibility
• avoids reflection, Unsafe, and JDK-internal shutdown hooks
• keeps closeGracefully() non-blocking
• adds deterministic Docker-free leak reproducer tests for both SSE and streamable HTTP transports

This is an alternative to the approaches explored in #610 and #868.

Motivation and Context

The root problem was that HttpClientSseClientTransport and HttpClientStreamableHttpTransport retained a strong reference to their internally created HttpClient even after transport close.

If user code retained closed transport objects, those internal HttpClient instances also remained reachable, which in turn kept HttpClient-* SelectorManager threads alive.

Instead of trying to forcibly shut down JDK HttpClient internals, this PR treats the issue as an ownership problem:

• transports own the internally created HttpClient
• once the transport is closed, that owned reference is released
• after that, the client becomes eligible for normal JVM cleanup

Additionally, DefaultMcpTransportSession.closeGracefully() now always disposes tracked connections even if onClose fails, which makes close-path cleanup reliable.

How Has This Been Tested?

Locally verified with targeted mcp-core regression tests:

• DefaultMcpTransportSessionTests
• HttpClientSseClientTransportLeakTests
• HttpClientStreamableHttpTransportLeakTests

These tests:

• repeatedly create and close transports
• intentionally keep closed transport objects reachable
• assert that HttpClient-* SelectorManager threads return to baseline after GC stabilization

Also verified with existing Docker-backed integration tests:

• HttpClientSseClientTransportTests
• HttpClientStreamableHttpTransportTest

Breaking Changes

None.

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 (https://modelcontextprotocol.io/)
• 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

Additional context

Implementation notes:

• Added an internal OwnedHttpClient abstraction to track transport-owned clients.
• HttpClientSseClientTransport and HttpClientStreamableHttpTransport now release their owned client reference on close.
• DefaultMcpTransportSession.closeGracefully() now disposes tracked connections even when onClose errors.
• The fix intentionally avoids reflection-based shutdown of JDK HttpClient internals.