fix: honor max turns in ACP sessions

[s-c-h-a-f-e-r]

ACP currently loads config.yaml but omits agent.max_turns when constructing AIAgent, leaving ACP/Buzz sessions capped at the constructor default of 90.

This passes a validated positive agent.max_turns as max_iterations while preserving the constructor default for missing or invalid values.

Verification:

• python -m pytest tests/acp/test_session.py -o "addopts=" -q (24 passed)
• python -m pytest tests/acp -o "addopts=" -q (135 passed, 2 pre-existing AsyncMock warnings)
• python -m compileall -q acp_adapter/session.py tests/acp/test_session.py
• git diff --check

[alt-glitch]

This was generated by AI during triage.

Duplicate of #70592. Both PRs implement the same validated agent.max_turns to ACP max_iterations propagation; #70592 was opened first and is the canonical implementation.

[s-c-h-a-f-e-r]

Closing as duplicate of #70592. The local regression coverage here independently confirmed the same ACP propagation gap: 135 ACP tests passed.