ACP 0.11 drops Gemini model state from NewSessionResponse

Problem

openhands-sdk declares agent-client-protocol>=0.10.1 without an upper bound, but ACP Python SDK 0.11.0 removes the unstable models field from NewSessionResponse.

Gemini CLI 0.46 still reports model state through that field and uses session/set_model. With ACP 0.11.0, Pydantic silently discards Gemini's models payload before _extract_session_models() sees it. The SDK then reports no current or available Gemini models:

assert cur == "gemini-3-flash-preview"
E   AssertionError: assert None == 'gemini-3-flash-preview'

This is separate from the prompt() argument-order incompatibility fixed by #3996. That fix is now on main, but the Gemini model-state failure remains, so downstream consumers still cannot safely remove their temporary agent-client-protocol<0.11 constraints.

Evidence

I ran the existing real-session-response regression suite, tests/sdk/agent/test_acp_agent.py, with both ACP versions:

The single failure in both 0.11.0 runs is:

FAILED tests/sdk/agent/test_acp_agent.py::TestDetectionAgainstRealSessionResponses::test_gemini_046_uses_set_session_model

The schema difference explains the result directly:

• ACP 0.10.1 NewSessionResponse.model_fields includes models.
• ACP 0.11.0 includes session_id, modes, config_options, and field_meta, but no models.
• Parsing the existing _GEMINI_046_SESSION fixture under 0.11.0 therefore drops models; _extract_session_models() returns (None, None, False).

Expected behavior

Either:

1. cap agent-client-protocol<0.11 in openhands-sdk until the incompatibility is resolved, or
2. preserve Gemini's model state through a local compatibility layer / move Gemini to the standard model configOptions mechanism.

Whichever route we take, every version allowed by the SDK dependency range should keep the existing Gemini model-detection regression test green.

References

• Prompt signature fix: #3996
• Existing real-provider response coverage and model-selection compatibility: #3773

Start with tests/sdk/agent/test_acp_agent.py, especially TestDetectionAgainstRealSessionResponses::test_gemini_046_uses_set_session_model, the _GEMINI_046_SESSION fixture, and _extract_session_models(). Compare parsing under agent-client-protocol 0.10.1 and 0.11.0, then determine whether the dependency cap or a compatibility approach preserves the existing regression coverage. Done means every version allowed by the SDK dependency range keeps the Gemini model-detection test green.