Lock collision-safe event ID contract and regression coverage

[davidahmann]

Problem

Event IDs must stay collision-safe across high-volume multi-agent/tool execution. Any shift toward normalized-name-only ID derivation would create ambiguous audit/replay trails.

Why Now

ADK usage is increasingly multi-agent and replay/audit-sensitive; this identifier contract should be explicitly protected by tests.

Current Behavior Is Insufficient

The implementation currently uses UUID-based IDs, but there is no focused regression test proving collision-safe behavior and non-dependence on normalized textual labels.

Why Code Change

This requires code-level tests (and hardening if needed) in event creation/conversion paths, not documentation guidance.

Scope / Codepaths

• src/google/adk/events/event.py
• src/google/adk/runners.py
• tests/unittests/a2a/converters/test_event_converter.py

Acceptance / Validation

• Add tests that validate generated event IDs are unique across collision-prone author/content naming corpus.
• Preserve existing interoperability and serialization behavior.
• Run changed-file formatter checks and targeted unit tests.

Repro Evidence Packet

• Version/commit under test: origin/main as of 2026-02-20.
• Environment: macOS (Darwin), Python/uv per repo setup.
• Minimal repro:
  1. Generate events for multiple agents/tool calls with normalized-equivalent names.
  2. Compare generated event IDs.
• Expected: IDs remain unique and independent of normalized labels.
• Actual: contract is implied but not explicitly guarded by focused regression tests.