feat(interactions): update ADK to support Google GenAI SDK v2.0.0

Update ADK to support the new Interactions API. See: https://ai.google.dev/gemini-api/docs/interactions-breaking-changes-may-2026

Key changes include:
* Updated `interactions_utils.py` to parse new v2 interaction events such as
  `InteractionCreatedEvent`, `InteractionCompletedEvent`, and explicit step
  types (`ModelOutputStep`, `FunctionCallStep`, etc.).
* Transitioned from "turns" to "steps" terminology, including renaming
  `convert_contents_to_turns` to `convert_contents_to_steps`.
* Updated `contributing/samples/interactions_api`

Breaking Changes in interactions_utils.py:

- Renamed "turn" functions to "step" equivalents:
  - `convert_content_to_turn` -> `_convert_content_to_step`
  - `convert_contents_to_turns` -> `_convert_contents_to_steps`
  - `convert_interaction_output_to_part` -> `_convert_interaction_step_to_parts`
- Updated `build_interactions_request_log` to use required `input_steps` instead of `input_turns`.
- Deprecated `convert_part_to_interaction_content`

Change-Id: I962724321342ba0192251896ce51832b214d60d9