docs(openapi): Add undocumented validate input endpoint (#2447)

### Description
- Add undocumented validate input endpoint

### Issues
Partially implements: #2286

Author: Pijukatel

apify-api/openapi/openapi.yaml

@@ -510,6 +510,8 @@ paths:
     $ref: "paths/actors/acts@{actorId}@run-sync.yaml"
   "/v2/acts/{actorId}/run-sync-get-dataset-items":
     $ref: "paths/actors/acts@{actorId}@run-sync-get-dataset-items.yaml"
+  "/v2/acts/{actorId}/validate-input":
+    $ref: "paths/actors/acts@{actorId}@validate-input.yaml"
   "/v2/acts/{actorId}/runs/{runId}/resurrect":
     $ref: "paths/actors/acts@{actorId}@runs@{runId}@resurrect.yaml"
   "/v2/acts/{actorId}/runs/last":

apify-api/openapi/paths/actors/acts@{actorId}@validate-input.yaml

@@ -0,0 +1,62 @@
+post:
+  tags:
+    - Actors
+  summary: Validate Actor input
+  description: |
+    Validates the provided input against the Actor's input schema for the specified build.
+
+    The endpoint checks whether the JSON payload conforms to the input schema
+    defined in the Actor's build. If no `build` query parameter is provided,
+    the `latest` build tag is used by default.
+  operationId: act_validateInput_post
+  parameters:
+    - $ref: "../../components/parameters/runAndBuildParameters.yaml#/actorId"
+    - name: build
+      in: query
+      description: |
+        Optional tag or number of the Actor build to use for input schema validation.
+        By default, the `latest` build tag is used.
+      required: false
+      style: form
+      explode: true
+      schema:
+        type: string
+        example: latest
+  requestBody:
+    description: JSON input to validate against the Actor's input schema.
+    content:
+      application/json:
+        schema:
+          type: object
+    required: true
+  responses:
+    "200":
+      description: ""
+      headers: {}
+      content:
+        application/json:
+          schema:
+            type: object
+            required:
+              - valid
+            properties:
+              valid:
+                type: boolean
+                description: Whether the input is valid according to the Actor's input schema.
+    "400":
+      $ref: ../../components/responses/BadRequest.yaml
+    "401":
+      $ref: ../../components/responses/Unauthorized.yaml
+    "403":
+      $ref: ../../components/responses/Forbidden.yaml
+    "404":
+      $ref: ../../components/responses/NotFound.yaml
+    "405":
+      $ref: ../../components/responses/MethodNotAllowed.yaml
+    "413":
+      $ref: ../../components/responses/PayloadTooLarge.yaml
+    "415":
+      $ref: ../../components/responses/UnsupportedMediaType.yaml
+    "429":
+      $ref: ../../components/responses/TooManyRequests.yaml
+  deprecated: false