feat(python): preserve unknown fields and open-union variants in Interactions models

Enable `preserveUnknownFields` and `modelSerializationMethods` in the Python SDK. Unknown fields are kept at every nesting level, in responses and in requests: unknown request fields are forwarded instead of silently dropped, so unsupported options such as `transcription_config.diarization` are rejected by the API rather than ignored (googleapis/python-genai#3162), and fields missing from the SDK schema reach the API. Unknown steps, content blocks and annotations round-trip as the payload the server sent, including stateless multi-turn history sent back as SDK objects or as stored dicts; known variants with invalid payloads still raise locally.

FUTURE_COPYBARA_INTEGRATE_REVIEW=#1371 from googleapis:release-please--branches--main dea37a4
PiperOrigin-RevId: 1009673588

Author: google-genai-bot

.github/workflows/unit-tests.yml

@@ -9,7 +9,7 @@ on:
       - main
 
 env:
-  SPEAKEASY_VERSION: v2.952.4
+  SPEAKEASY_VERSION: v2.955.0
 
 jobs:
   unit-test: