fix interactions SSE ids for function calls

[kshirajahere]

What changed

• preserve interaction chain IDs in Interactions API SSE mode by reading the SDK's actual streaming fields (interaction.id, interaction_id, event_id) instead of only event.id
• treat interaction.complete as the final full interaction event during streaming conversion
• add regression coverage for interaction.complete handling and for SSE function-call streams that must retain the interaction ID before the tool round-trip

Why

Issue #5169 reports that StreamingMode.SSE breaks Interactions API function calling because the second model call cannot find the previous interaction_id in session history.

The root cause is that ADK's SSE adapter was keyed to older/generic event shapes (event.id and event_type == "interaction"), while google-genai 1.70.0 emits interaction.start, interaction.complete, and interaction.status_update events with IDs stored on interaction.id, interaction_id, and event_id.

That mismatch meant streamed function-call events were persisted without an interaction_id, so the next round of the ReAct loop could not chain the interaction and failed with invalid_request.

Impact

• restores function-calling flows for Gemini(use_interactions_api=True) when SSE streaming is enabled
• keeps the interaction chain intact in session events so follow-up tool/result turns can resume correctly
• improves compatibility with the current GenAI SDK SSE event schema without changing non-streaming behavior

Validation

• .venv\\Scripts\\pytest tests\\unittests\\models\\test_interactions_utils.py tests\\unittests\\flows\\llm_flows\\test_interactions_processor.py tests\\unittests\\models\\test_google_llm.py -q
• .venv\\Scripts\\pyink --check --config pyproject.toml src\\google\\adk\\models\\interactions_utils.py tests\\unittests\\models\\test_interactions_utils.py
• .venv\\Scripts\\isort --check-only src\\google\\adk\\models\\interactions_utils.py tests\\unittests\\models\\test_interactions_utils.py
• .venv\\Scripts\\pylint --rcfile=pylintrc src\\google\\adk\\models\\interactions_utils.py

[kshirajahere]

Local validation is green on this branch:\n\n- pytest tests/unittests/models/test_interactions_utils.py tests/unittests/flows/llm_flows/test_interactions_processor.py tests/unittests/models/test_google_llm.py -q\n- pyink --check --config pyproject.toml src/google/adk/models/interactions_utils.py tests/unittests/models/test_interactions_utils.py\n- isort --check-only src/google/adk/models/interactions_utils.py tests/unittests/models/test_interactions_utils.py\n- pylint --rcfile=pylintrc src/google/adk/models/interactions_utils.py\n- local check-file-contents verification against origin/main\n- no new mypy error lines relative to origin/main for src/google/adk/models/interactions_utils.py\n\nThe remaining GitHub Actions jobs are currently in �ction_required because this is a forked PR and upstream workflow approval is needed before they can actually run.