feat(py/plugins/google-genai): Interactions API support

[cabljac]

Dedicated tracker for Python Interactions API support in py/plugins/google-genai, under the cross-SDK umbrella #5804.

Work is in flight as a six-PR stack (in review):

• feat(py): core support for background model operations and typed model refs #5854 - core support for background model operations
• feat(py/plugins/google-genai): raw Interactions HTTP client, converters, and options #5855 - raw Interactions HTTP client
• feat(py/plugins/google-genai): Deep Research, Antigravity, and Lyria interaction models #5856 - Deep Research / Antigravity / Lyria models
• refactor(py/plugins/google-genai): move Veo to background model actions #5857 - Veo as background actions
• feat(py/plugins/google-genai): wire Interactions models into GoogleAI plugin #5858 - GoogleAI wiring
• feat(py/plugins/google-genai): finish_message for image models and effective config echo #5859 - finish_message and config echo

Plus the SDK bump #5839. Follows the JS reference (#5317): new models route through Interactions, existing models stay on generateContent, per-model boolean routing. Vertex is out of scope - gated on the Vertex Interactions rollout (see #5804). Prior art #5798 closed unmerged.

Related: #5692 (Python background-models surface) - coordinate to avoid overlap.