feat!: Engine V7 compatibility

[khvn26]

Closes #144, #145.

In this PR, we:

• Drop Python 3.8 support.
• Drop the Pydantic dependency.
• Stop respecting hide_disabled_flags in Local evaluation mode.
• Eliminate all usage of Engine v6's Pydantic models and fully switch to EvaluationContext and EvaluationResult usage.
• Introduce the new OfflineHandler signature, add a new evaluation context-based LocalOfflineHandler, and preserve an environment document-based one.
• Map identity overrides from the environment document to segments in the engine evaluation context.

This makes the Python SDK fully compatible with next major version of the engine and fully enables Context values in local evaluation mode.

[emyller]

Looks good with non-blocking comments.

[emyller]

Thanks for picking every nit!