Validate session initialization events

[petrmarinec]

Summary

Fixes #5290.

This change prevents client-supplied session initialization events from seeding ADK runtime/tool protocol state. Session initialization still accepts non-runtime conversation history, but rejects events containing function calls, function responses, long-running tool ids, or internal runtime EventActions fields.

It also tightens HITL confirmation resumption so adk_request_confirmation responses only resume tool calls that are backed by prior original function-call state and prior requested_tool_confirmations metadata in the event history.

Why

CreateSessionRequest.events is accepted from the HTTP API and appended to the new session. Those events should not be able to spoof ADK-generated runtime control state. Without this validation, a client can seed confirmation-shaped events and later send a matching confirmation response that causes a registered tool call embedded in originalFunctionCall to be resumed.

Tests

• python -m pyink --check src/google/adk/flows/llm_flows/request_confirmation.py src/google/adk/cli/adk_web_server.py tests/unittests/flows/llm_flows/test_request_confirmation.py tests/unittests/cli/test_fast_api.py
• python -m pytest tests/unittests/flows/llm_flows/test_request_confirmation.py tests/unittests/cli/test_fast_api.py::test_create_session_accepts_initial_text_events tests/unittests/cli/test_fast_api.py::test_create_session_rejects_runtime_tool_events tests/unittests/cli/test_fast_api.py::test_create_session_rejects_runtime_action_events tests/unittests/runners/test_run_tool_confirmation.py -q

Result: 17 passed.

Additional local check: tests/unittests/cli/test_fast_api.py passes on Windows with known baseline-local failures excluded: 61 passed, 4 deselected. The excluded cases are two CRLF assertion failures in builder GET tests and two A2A temporary-directory teardown errors; I reproduced those same failures on clean origin/main.

Checklist

• I have read the CONTRIBUTING.md document.
• I have performed a self-review of my own code.
• I have added tests that prove my fix is effective.
• New and existing targeted unit tests pass locally with my changes; unrelated Windows baseline failures are documented above.