docs(openapi): remove top-level width/height fields on Asset schema (#13973)

These two fields were added recently to the Asset schema as nullable
integers, with the intent of exposing original image dimensions for FE
consumers (cloud-side thumbnailing makes naturalWidth/Height return
the wrong size for an image card's dimension label).

The implementation effort that consumes them subsequently converged on
a different shape — dimensions nested under the existing free-form
`metadata` JSON field as `{kind: "image", width, height}` — to avoid
introducing type-specific flat fields on the canonical Asset shape,
and to leave room for forward-compatible additions (video duration,
fps, etc.) without further schema churn.

This removes the now-unused top-level fields so the spec reflects the
agreed direction. No other schema definitions reference these fields
directly: AssetCreated, AssetUpdated, etc. inherit Asset via allOf and
do not redefine them.

The runtime ingest implementation that would have populated these
fields was not yet shipped, so no clients are relying on the
top-level shape.

Co-authored-by: Alexis Rolland <alexisrolland@hotmail.com>

openapi.yaml

@@ -6351,14 +6351,6 @@ components:
           type: integer
           format: int64
           description: Size of the asset in bytes
-        width:
-          type: integer
-          nullable: true
-          description: "Original image width in pixels. Null for non-image assets or assets ingested before dimension extraction."
-        height:
-          type: integer
-          nullable: true
-          description: "Original image height in pixels. Null for non-image assets or assets ingested before dimension extraction."
         mime_type:
           type: string
           description: MIME type of the asset