feat(dotnet): expand implicit union conversions and mark Interactions types partial

Expand implicit conversion operators across Interactions union models—including nested union member conversions (such as passing `TextContent` or `ImageContent` directly to `InteractionsInput`, or `TextResponseFormat` directly to `CreateModelInteractionResponseFormat` and `CreateAgentInteractionResponseFormat`), `string` to `Step` (`UserInputStep`), and null-propagating (`[NotNullIfNotNull]`) conversions—and declare Interactions model, client, and exception types as `partial`.

PiperOrigin-RevId: 1013922181

Author: Google Team Member

.github/workflows/ci.yaml

@@ -10,7 +10,7 @@ permissions:
     contents: read
 
 env:
-    SPEAKEASY_VERSION: v2.959.1
+    SPEAKEASY_VERSION: v2.960.7
 
 jobs:
     non-live-validation:

internal/cli/custom/custom_test.go

@@ -2563,4 +2563,3 @@ func TestMergeMediaBlocksIntoInput(t *testing.T) {
 		})
 	}
 }
-