fix(aionui-ai-agent): preserve ACP internal error payloads (AIO-101)

[TCP404]

Summary

• Treat ACP JSON-RPC -32603 as a generic Agent internal error instead of letting lifecycle/provider heuristics hide the SDK payload.
• Preserve concrete SDK error data such as Failed to connect MCP servers in stream error detail for frontend display/diagnostics.
• Prefer populated -32603 payloads over later process-exit metadata in ACP close reason; exit code remains the fallback when no useful payload exists.

Multica Issue

AIO-101

Verification

• cargo fmt --check
• cargo test -p aionui-ai-agent
• cargo clippy -p aionui-ai-agent -- -D warnings