feat: expose BrandManifestReference union type in stable API

Add BrandManifestReference as a union type (BrandManifest | AnyUrl) to
properly represent the oneOf schema from brand-manifest-ref.json. This
allows users to pass either an inline BrandManifest object or a URL
string pointing to a hosted manifest.

Changes:
- Add BrandManifestReference = BrandManifest1 | AnyUrl in stable.py
- Export BrandManifestReference from adcp.types
- Restore deprecation warning to generated_poc/__init__.py

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

Author: bokelley

src/adcp/types/__init__.py

@@ -36,6 +36,7 @@
 # Import stable public API types
 from adcp.types.stable import (
     BrandManifest,
+    BrandManifestReference,
     Creative,
     CreativeStatus,
     Error,
@@ -86,6 +87,7 @@
     "UrlVastAsset",
     # Stable API types (commonly used)
     "BrandManifest",
+    "BrandManifestReference",
     "Creative",
     "CreativeStatus",
     "Error",

src/adcp/types/generated_poc/__init__.py

@@ -1,6 +1,6 @@
 # generated by datamodel-codegen:
 #   filename:  .schema_temp
-#   timestamp: 2025-11-18T03:35:10+00:00
+#   timestamp: 2025-11-18T04:03:30+00:00
 
 """Generated AdCP types - INTERNAL USE ONLY.
 

src/adcp/types/stable.py

@@ -21,6 +21,8 @@
 
 from __future__ import annotations
 
+from pydantic import AnyUrl
+
 # Import all generated types
 from adcp.types.generated import (
     # Core request/response types
@@ -98,6 +100,11 @@
 # These provide a clean public API that hides schema evolution details
 BrandManifest = BrandManifest1
 
+# Union types for oneOf schemas
+# BrandManifestReference represents the union from brand-manifest-ref.json:
+# either an inline BrandManifest object OR a URL string pointing to a hosted manifest
+BrandManifestReference = BrandManifest1 | AnyUrl
+
 # Re-export all stable types
 __all__ = [
     # Request/Response types
@@ -133,6 +140,7 @@
     "UpdateMediaBuyResponse",
     # Domain types
     "BrandManifest",  # Stable alias for BrandManifest1
+    "BrandManifestReference",  # Union type: BrandManifest | AnyUrl
     "Creative",
     "CreativeManifest",
     "Error",