docs: clarify RunResult.events testing surface

[nightcityblade]

Fixes #5410

Documents RunResult.events as a small, testing-oriented surface, enumerates the event types it contains, and clarifies that callers should use it for run assertions instead of depending on lower-level session internals or raw media artifacts.

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[devin-ai-integration]

✅ Devin Review: No Issues Found

Devin Review analyzed this PR and found no bugs or issues to report.