[codex] fix MCP streamable HTTP close flake

[hengyunabc]

Summary

Fix a flaky MCP Streamable HTTP integration test failure where HttpURLConnection can reuse a socket that the server has already closed after initialize, causing java.net.SocketException: Unexpected end of file from server in ArthasMcpToolsIT on CI.

Root cause

The streamable HTTP server writes several ordinary JSON/empty HTTP responses and immediately closes the Netty channel with ChannelFutureListener.CLOSE, but those responses did not advertise Connection: close. Under HTTP/1.1, the client can treat the socket as reusable and send the next MCP POST (notifications/initialized) on a closing connection.

Changes

• Add Connection: close to MCP server responses that close the channel after non-SSE responses.
• Add Connection: close to top-level MCP error responses.
• Harden handwritten MCP HttpURLConnection test clients to request close, disconnect after each response, and retry notifications/initialized on transient IOException.

Validation

• git diff --check
• mvn -V -ntp -pl arthas-mcp-integration-test -am -Dtest=com.taobao.arthas.mcp.it.ArthasMcpToolsIT -Dsurefire.failIfNoSpecifiedTests=false verify
• mvn -V -ntp -pl arthas-mcp-integration-test verify

[Copilot]

Pull request overview

Fixes a flaky MCP streamable HTTP integration-test failure caused by the server closing HTTP/1.1 connections without advertising Connection: close, allowing HttpURLConnection to reuse a socket that the server is already closing.

Changes:

• Add Connection: close to MCP server HTTP responses that immediately close the Netty channel (including error responses).
• Update handwritten HttpURLConnection test clients to request Connection: close and always disconnect() after each request.
• Add a small retry/backoff for notifications/initialized to tolerate transient IOException during connection teardown.

Reviewed changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated no comments.

File	Description
arthas-mcp-server/src/main/java/com/taobao/arthas/mcp/server/protocol/server/handler/McpStreamableHttpRequestHandler.java	Adds Connection: close to non-SSE responses and error responses that close the channel immediately.
arthas-mcp-server/src/main/java/com/taobao/arthas/mcp/server/protocol/server/handler/McpHttpRequestHandler.java	Adds Connection: close to top-level HTTP error responses that close the channel.
arthas-mcp-integration-test/src/test/java/com/taobao/arthas/mcp/it/task/ArthasMcpTaskTestSupport.java	Hardens integration-test HTTP client behavior (request close, disconnect in finally, retry initialized notification).
arthas-mcp-integration-test/src/test/java/com/taobao/arthas/mcp/it/ArthasMcpToolsIT.java	Applies the same HTTP client hardening to the ArthasMcpToolsIT test client.

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