fix(mcp): support mcp 2 streamable http client

[dvd233]

Summary

• Support the renamed streamable_http_client transport in mcp 2.x while keeping mcp 1.x compatibility.
• Configure mcp 2.x httpx2 headers and connect/read timeouts, and close the client through the existing AsyncExitStack.
• Add a regression assertion for both transport API names.

Fixes #119

Validation

• mcp 1.9.4 focused MCP tests: 24 passed, 1 skipped.
• mcp 2.0.0 focused MCP tests: 24 passed, 1 skipped.
• python -m compileall -q mini_agent tests/test_mcp.py
• git diff --check

The focused test selection excludes the repository's optional Git MCP tests and Windows-specific temporary-file cleanup cases; the latter still fail independently on this host because the existing tests unlink an open NamedTemporaryFile.

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