fix(cogvideo_video): consult variant i2v flag instead of advertising it unconditionally

COGVIDEO_VARIANTS declares cogvideo-2b i2v=False (it is t2v-only), but
cogvideo_video advertised image_to_video + reference_image unconditionally
and the variant flag was never consulted. An image_to_video brief against
the 2B variant reached the diffusion pipeline and failed opaquely.

- Add is_operation_available(operation) that derives capability from the
  variant table (the selector calls it without inputs, so it reports the
  DEFAULT variant cogvideo-5b: t2v + i2v both True). This replaces an
  implicit unconditional-True.
- Add an execute()-time guard that consults the CALLER's chosen variant
  and fails fast with a clear error when it lacks the requested mode
  (2B + image_to_video), instead of dropping into generate_local_video.
- Add _variant_for(inputs) helper shared by estimate_runtime / the guard.

Tests pin: the 2B premise (i2v=False), default-variant capability
reporting, fast-fail for 2B+i2v (generation never runs), and that 5B+i2v
still routes through to generate_local_video.

Refs: docs/REVIEW-image-to-video-voice.md §8 #4

Co-Authored-By: Claude <noreply@anthropic.com>

tests/tools/test_cogvideo_i2v_variant.py

@@ -0,0 +1,78 @@
+"""Regression for cogvideo-2b i2v mismatch (REVIEW §8 #4).
+
+COGVIDEO_VARIANTS declares the 2B variant i2v=False (t2v-only), but
+cogvideo_video advertised image_to_video + reference_image unconditionally
+and never consulted the variant flag — so an image_to_video brief against
+cogvideo-2b reached the diffusion pipeline and failed opaquely.
+
+This pins:
+- is_operation_available() derives capability from the variant table
+  (default 5b: t2v + i2v both True), not an unconditional True.
+- execute() fails fast with a clear error when the caller's chosen variant
+  lacks the requested mode (2b + image_to_video).
+"""
+
+from __future__ import annotations
+
+import pytest
+
+from tools.base_tool import ToolStatus
+from tools.video._shared import COGVIDEO_VARIANTS
+from tools.video.cogvideo_video import CogVideoVideo
+
+
+def test_2b_variant_is_t2v_only():
+    """The premise: the 2B variant really does declare i2v=False."""
+    assert COGVIDEO_VARIANTS["cogvideo-2b"]["i2v"] is False
+    assert COGVIDEO_VARIANTS["cogvideo-2b"]["t2v"] is True
+
+
+def test_5b_variant_supports_i2v():
+    assert COGVIDEO_VARIANTS["cogvideo-5b"]["i2v"] is True
+
+
+def test_is_operation_available_reflects_default_variant():
+    tool = CogVideoVideo()
+    # Default variant (5b) supports both modes.
+    assert tool.is_operation_available("text_to_video") is True
+    assert tool.is_operation_available("image_to_video") is True
+
+
+def test_execute_rejects_i2v_for_2b_variant(monkeypatch):
+    """Selecting the 2B variant for image_to_video fails fast, not at the pipeline."""
+    monkeypatch.setattr(
+        CogVideoVideo, "get_status", lambda self: ToolStatus.AVAILABLE
+    )
+    # Guard against the local-generation path actually running.
+    monkeypatch.setattr(
+        "tools.video.cogvideo_video.generate_local_video",
+        lambda **kw: pytest.fail("generate_local_video must not run for an unsupported variant"),
+    )
+
+    result = CogVideoVideo().execute(
+        {"prompt": "x", "operation": "image_to_video", "model_variant": "cogvideo-2b"}
+    )
+    assert result.success is False
+    assert "cogvideo-2b" in result.error
+    assert "image_to_video" in result.error
+
+
+def test_execute_allows_i2v_for_5b_variant(monkeypatch):
+    """The default 5B variant still routes to generation for image_to_video."""
+    from tools.base_tool import ToolResult
+
+    monkeypatch.setattr(
+        CogVideoVideo, "get_status", lambda self: ToolStatus.AVAILABLE
+    )
+
+    sentinel = ToolResult(success=True, data={"output": "out.mp4"})
+
+    def fake_generate(**kw):
+        return sentinel
+
+    monkeypatch.setattr("tools.video.cogvideo_video.generate_local_video", fake_generate)
+
+    result = CogVideoVideo().execute(
+        {"prompt": "x", "operation": "image_to_video", "model_variant": "cogvideo-5b"}
+    )
+    assert result.success is True