fix(adk): bump ag-ui-protocol minimum to >=0.1.15 for multimodal types

The new multimodal input types (ImageInputContent, AudioInputContent,
etc.) were added in ag-ui-protocol 0.1.15. Without this pin, CI
installs an older version from PyPI which lacks these types.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: contextablemark

integrations/adk-middleware/python/pyproject.toml

@@ -8,7 +8,7 @@ authors = [
 ]
 requires-python = ">=3.10, <3.15"
 dependencies = [
-    "ag-ui-protocol>=0.1.11",
+    "ag-ui-protocol>=0.1.15",
     "aiohttp>=3.12.0",
     "asyncio>=3.4.3",
     "fastapi>=0.115.2",