fix(screencast): throw if start() is called without onFrame handler

[Kikobeats]

Summary

• Throw a clear error when start() is called before registering an onFrame callback, instead of silently dropping frames
• Update tests to register onFrame before start() where missing

Context

If start() was called before onFrame(), CDP frames were acknowledged but never delivered to the caller, with no error or warning. This makes the API fail fast with an actionable message.

Closes #698

Made with Cursor

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Note

Low Risk
Low risk: adds a fail-fast runtime check and adjusts tests; behavior only changes for incorrect caller usage (calling start() without registering onFrame).

Overview
packages/screencast now throws a clear error if start() is called before an onFrame callback is registered, instead of starting the CDP screencast and silently dropping frames.

Tests are updated to always register a no-op onFrame handler before starting screencast sessions, keeping the listener cleanup assertions intact.

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[coveralls]

coverage: 74.717% (-0.02%) from 74.733% — fix/screencast-require-onframe into master