test: add env var filtering test and use --mcp-config flag
- Add mcp_config option to ServerProcess for --mcp-config flag
- Refactor tests to use server.mcp_config instead of env vars
- Add get_env_vars tool to echo server for security testing
- Add TestMcpEnvVarFiltering test class to verify:
- Secret env vars (API keys, passwords) are NOT passed to subprocess
- Allowed env vars (HOME, PATH, etc.) ARE inherited
- Config-specified env vars ARE passed through
- All 8 MCP tests now pass
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Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>
