fix(theme): handle nested discriminators in allOf structures (#1303)

* fix(theme): handle nested discriminators in allOf structures

Add support for discriminators nested within allOf schemas. Previously,
discriminators inside allOf items were not properly detected, and the
discriminator property lookup only checked top-level schema properties.

Changes:
- Add findProperty() helper to recursively search for properties in
  nested oneOf/anyOf/allOf structures
- Update DiscriminatorNode to use findProperty() for discriminator
  property lookup
- Update SchemaNode to check for discriminators in allOf items and
  merge schemas before processing
- Return null instead of empty "object" SchemaItem when properties
  are empty after discriminator processing
- Add test case for nested discriminator in allOf

This fixes scenarios like:
```yaml
allOf:
  - $ref: CommonProps
  - oneOf: [VariantA, VariantB]
    discriminator:
      propertyName: type
```

Fixes #1302

* fix(theme): improve nested discriminator handling and examples

Handle recursively nested discriminators in composed schemas while preserving intentional empty-object rendering, and add demo specs covering deep allOf discriminator chains and explicit empty object properties.

Co-authored-by: Cursor <cursoragent@cursor.com>

---------

Co-authored-by: Cursor <cursoragent@cursor.com>

Author: sserrata

demo/examples/tests/allOf.yaml

@@ -510,6 +510,39 @@ paths:
                   name:
                     type: string
 
+  /allof-empty-object-properties:
+    get:
+      tags:
+        - allOf
+      summary: allOf with explicit empty object properties
+      description: |
+        Demonstrates an intentional empty object schema (properties: {}) to ensure
+        the renderer still shows the object shape rather than hiding it.
+
+        Schema:
+        ```yaml
+        type: object
+        properties:
+          metadata:
+            type: object
+            properties: {}
+          name:
+            type: string
+        ```
+      responses:
+        "200":
+          description: Successful response
+          content:
+            application/json:
+              schema:
+                type: object
+                properties:
+                  metadata:
+                    type: object
+                    properties: {}
+                  name:
+                    type: string
+
   /allof-multiple-oneof:
     post:
       tags: