fix(acp): improve vendor compatibility for session/update parsing

[chenhg5]

Summary

• Added debug logging to capture raw session/update JSON for troubleshooting vendor compatibility issues
• Enhanced mapAgentMessageChunk() to support multiple JSON formats:
  • Standard ACP: {"content": {"type": "text", "text": "..."}}
  • Alternative: {"content": {"text": "..."}} (type omitted)
  • Alternative: {"text": "..."} (top-level text field)
  • Alternative: {"content": "..."} (content as string)
• Improved mapSessionUpdateFallback() to extract text from common vendor extensions (message, message_chunk, response, text)
• Added comprehensive tests for all alternative formats

Root Cause

Issue #432 reported "(empty response)" when using OpenClaw/OpenCode with cc-connect. The ACP agent was only parsing the standard content.text format. If a vendor sends text in a different JSON structure, the mapping function returned nil, resulting in no EventText events and eventually "(empty response)" in the UI.

Test plan

• Build passes: go build ./...
• Tests pass: go test ./...
• Manual test: use OpenClaw/OpenCode with Feishu and verify responses are displayed correctly

Fixes #432

🤖 Generated with Claude Code

[chenhg5]

LGTM! Clean fix for vendor compatibility.

✅ Correctness: Multiple JSON format fallbacks correctly handle various vendor implementations

• Standard ACP format
• content.text without type
• Top-level text field
• Content as string

✅ Tests: Comprehensive test coverage for all alternative formats

✅ Debug logging: Added slog.Debug statements for troubleshooting

✅ CI: All checks pass

✅ Documentation: Added OpenClaw pairing requirements to config.example.toml

Fixes #432 - OpenClaw/OpenCode empty response issue.