fix: support agent peers on ADK 2.0 and 2.1 - #1047

Summary

• backport ADK 2.2 peer-transfer compatibility for ADK 2.0 and 2.1
• provide a non-serialized mode=None fallback through BaseAgent, covering Parallel, Sequential, Loop, and custom agents
• keep ADK 1.x and ADK 2.2+ behavior unchanged
• add real-runner transfer coverage with deeply nested mixed workflow agents, plus version-boundary and idempotency tests

Tests

• pre-commit run --all-files
• ADK 2.0.0: 18 passed
• ADK 2.1.0: 18 passed
• ADK 2.2.0: 17 passed, 1 skipped
• full suite on ADK 2.2.0: 3404 passed, 13 skipped, 2 xfailed, 6 subtests passed