feat(coderd): add task prompt modification endpoint (#20811)

This PR adds the backend implementation for modifying task prompts. Part
of https://github.com/coder/internal/issues/1084

## Changes

- New `UpdateTaskPrompt` database query to update task prompts
- New PATCH `/api/v2/tasks/{task}/prompt` endpoint

## Notes

This is part 1 of a 2-part PR stack. The frontend UI will be added in a
follow-up PR based on this branch
(https://github.com/coder/coder/pull/20812).

---

🤖 PR was written by Claude Sonnet 4.5 Thinking using [Coder
Mux](https://github.com/coder/cmux) and reviewed by a human 👩

coderd/database/modelmethods.go

@@ -132,11 +132,28 @@ func (w ConnectionLog) RBACObject() rbac.Object {
 	return obj
 }
 
+// TaskTable converts a Task to it's reduced version.
+// A more generalized solution is to use json marshaling to
+// consistently keep these two structs in sync.
+// That would be a lot of overhead, and a more costly unit test is
+// written to make sure these match up.
+func (t Task) TaskTable() TaskTable {
+	return TaskTable{
+		ID:                 t.ID,
+		OrganizationID:     t.OrganizationID,
+		OwnerID:            t.OwnerID,
+		Name:               t.Name,
+		WorkspaceID:        t.WorkspaceID,
+		TemplateVersionID:  t.TemplateVersionID,
+		TemplateParameters: t.TemplateParameters,
+		Prompt:             t.Prompt,
+		CreatedAt:          t.CreatedAt,
+		DeletedAt:          t.DeletedAt,
+	}
+}
+
 func (t Task) RBACObject() rbac.Object {
-	return rbac.ResourceTask.
-		WithID(t.ID).
-		WithOwner(t.OwnerID.String()).
-		InOrg(t.OrganizationID)
+	return t.TaskTable().RBACObject()
 }
 
 func (t TaskTable) RBACObject() rbac.Object {