fix: filter non-JSON log messages from ACP agent output

[gandli]

Problem

acpx fails to integrate with iflow CLI due to JSON parse errors when the agent outputs log messages like:

[iFlow ACP Agent] ACP adapter factory initialized
Failed to parse JSON message: [iFlow ACP Agent] ACP adapter factory initialized
SyntaxError: Unexpected token 'i', "[iFlow ACP "... is not valid JSON

Root Cause

iflow CLI outputs initialization log messages to stdout/stderr during startup. These non-JSON lines are passed to @agentclientprotocol/sdk's ndJsonStream parser, which attempts to parse them as JSON and fails.

Solution

Added two-layer filtering to prevent non-JSON log messages from breaking the ACP protocol stream:

1. filterAcpOutputStream() - New method that filters the output stream before ndJsonStream parsing, with:
   • Fail-fast mechanism: throws error after 10 consecutive non-JSON lines
   • EOF buffer flush: handles final JSON-RPC frame without trailing newline
2. isValidAcpMessage() - Additional validation layer in createTappedStream()

Testing

• ✅ All 253 existing tests pass
• ✅ acpx iflow sessions new now works without JSON parse errors
• ✅ Compatible with other ACP agents that output log messages

Impact

Fixes integration with iflow CLI and other ACP agents that output initialization log messages to stdout/stderr during startup.

Codex Review Addressed

• ✅ P2: Fail fast on stdout protocol violations - Added error after 10 consecutive non-JSON lines
• ✅ P2: Flush the buffered final frame at EOF - Buffer is now flushed when stream closes

[chatgpt-codex-connector]

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