Make stream parsing backend-aware

[shinpr]

Summary

• bind stream parsing to the configured backend instead of inferring protocols from output events
• validate backend-specific terminal results and preserve agent errors as errors
• process stdout incrementally without repeatedly scanning a growing incomplete line
• bump the package and server metadata version to 0.12.1

Verification

• npm run check:all (396 tests passed)
• real CLI smoke tests through AgentExecutor returned SMOKE_OK for Claude, Codex, Gemini, Grok, and OpenCode
• Claude-compatible terminal error responses were normalized as errors during GLM and Kimi smoke attempts
• Cursor reached the real CLI but could not complete because the local environment has no Cursor authentication configured

All smoke tests invoked AgentExecutor directly, exercising the production stream parsing path without the MCP transport layer.