feat: update capability-backed draft steps

src/wf_api/draft_updates.py

@@ -0,0 +1,26 @@
+from __future__ import annotations
+
+from typing import Self
+
+from pydantic import BaseModel, ConfigDict, Field, model_validator
+
+from wf_core.models.steps import InputBinding
+
+
+class CapabilityStepUpdate(BaseModel):
+    """Presence-aware patch for one existing capability-backed draft step."""
+
+    model_config = ConfigDict(extra="forbid")
+
+    desc: str | None = Field(default=None, min_length=1)
+    retry: int | None = Field(default=None, ge=0)
+    timeout_seconds: int | None = Field(default=None, gt=0)
+    input: list[InputBinding] | None = None
+
+    @model_validator(mode="after")
+    def validate_patch_shape(self) -> Self:
+        if not self.model_fields_set:
+            raise ValueError("capability step update requires at least one field")
+        if "input" in self.model_fields_set and self.input is None:
+            raise ValueError("capability step update input must be a list")
+        return self