fix: support MCP 2 transport APIs

[rohitsalla]

Summary

• use the MCP 2.x streamable_http_client API while retaining the MCP 1.x fallback
• register the legacy WebSocket transport only when the installed MCP SDK provides it
• exercise the core transport integration tests against MCP 2.x in CI
• make the SSE and Streamable HTTP test servers compatible with both MCP server APIs

Root cause

MCP 2.0 renamed streamablehttp_client to streamable_http_client and removed the non-standard WebSocket client module. Importing either legacy symbol unconditionally prevented mcpadapt.core from loading, including for unaffected stdio, SSE, and Streamable HTTP users.

Validation

• MCP 1.x core suite: 18 passed
• MCP 2.x applicable core suite: 14 passed, 4 WebSocket tests deselected
• ruff check src/mcpadapt/core.py tests/test_core.py
• ruff format --check src/mcpadapt/core.py tests/test_core.py

Fixes #88