fix(video): runway/higgsfield model defaults diverge from schema

runway (estimate_cost/estimate_runtime/execute) defaulted to gen4_turbo and
higgsfield (execute) defaulted to kling_3.0, while both schemas advertise
seedance_2.0 as model.default. Omitting model under-quoted cost (runway 6x:
$0.25 vs $1.50) and silently generated a different model than advertised,
violating the Decision-Communication / cost-accuracy contract.

Root cause was a default duplicated across schema + 3 methods that drifted.
Collapse it to a single _DEFAULT_MODEL constant referenced everywhere.

Add tests/tools/test_provider_model_defaults.py to lock each tool's
estimate default to its schema default and guard the execute path.

tests/tools/test_provider_model_defaults.py

@@ -0,0 +1,44 @@
+"""Regression: a provider's code-level model default must match its schema's
+declared `model.default`.
+
+Bug: runway_video and higgsfield_video hardcoded stale model defaults in
+estimate_cost/estimate_runtime/execute (`gen4_turbo` / `kling_3.0`) while the
+schema advertised `seedance_2.0` as the premium default. Omitting `model` then
+quoted the wrong (cheap) model's cost and silently generated a different model
+than the schema promised — a Decision-Communication / cost-accuracy violation.
+"""
+
+import pytest
+
+import tools.video.higgsfield_video as higgsfield_video
+import tools.video.runway_video as runway_video
+from tools.video.higgsfield_video import HiggsFieldVideo
+from tools.video.runway_video import RunwayVideo
+
+
+@pytest.mark.parametrize(
+    "tool_cls, module",
+    [(RunwayVideo, runway_video), (HiggsFieldVideo, higgsfield_video)],
+)
+def test_default_model_constant_matches_schema(tool_cls, module):
+    # `execute()` reads `model` via `_DEFAULT_MODEL`; locking the constant to the
+    # schema default guards the silent-model-swap path without a network call.
+    schema_default = tool_cls().input_schema["properties"]["model"]["default"]
+    assert module._DEFAULT_MODEL == schema_default
+
+
+@pytest.mark.parametrize("tool_cls", [RunwayVideo, HiggsFieldVideo])
+def test_estimate_default_model_matches_schema(tool_cls):
+    tool = tool_cls()
+    schema_default = tool.input_schema["properties"]["model"]["default"]
+
+    # Cost/runtime with `model` omitted must equal the schema's declared default,
+    # not some stale hardcoded fallback.
+    assert tool.estimate_cost({}) == tool.estimate_cost({"model": schema_default}), (
+        f"{tool.name}.estimate_cost default model diverges from schema default "
+        f"{schema_default!r}"
+    )
+    assert tool.estimate_runtime({}) == tool.estimate_runtime({"model": schema_default}), (
+        f"{tool.name}.estimate_runtime default model diverges from schema default "
+        f"{schema_default!r}"
+    )