perf: cache SDK version metadata

[Linxiushen]

Summary

• cache the installed OpenHands SDK version for the process lifetime
• avoid repeated synchronous package-metadata filesystem lookups on every telemetry event
• add a regression test that verifies repeated reads perform one metadata lookup

Testing

• pytest tests/test_version.py --noconftest -q
• ruff check openhands/automation/utils/version.py tests/test_version.py
• ruff format --check openhands/automation/utils/version.py tests/test_version.py

Fixes #289

Disclosure: This change was prepared with AI assistance. I reviewed the implementation and ran the checks listed above.