fix(schemas): preserve multivariate option key in DynamoDB document

[gagantrivedi]

Problem

When a multivariate option is created with a key, the key is correctly threaded into the engine model (MultivariateFeatureOptionModel.key) in flagsmith core, but it never reaches the DynamoDB environment document. Documents end up like:

"multivariate_feature_option": {
  "id": 29176,
  "value": "variant_1"
}

— no key.

Root cause

The environment document is validated against EnvironmentCompressed during compression (map_environment_to_compressed_environment_document → TypeAdapter(EnvironmentCompressed).validate_python(...)). The MultivariateFeatureOption TypedDict here only declared id and value, so Pydantic silently dropped the extra key key during validation.

Fix

Add key: NotRequired[str | None] to the MultivariateFeatureOption TypedDict so it survives serialisation. Nullable + not-required keeps backward compatibility with documents written before keys existed.

Testing

• Added a regression test asserting key is preserved through TypeAdapter(MultivariateFeatureOption).validate_python(...).
• The originating bug is also covered by a failing test in flagsmith core (test_map_environment_to_compressed_environment_document__mv_option_with_key__key_preserved), which will pass once this is released and the dependency bumped.

[gemini-code-assist]

Code Review

This pull request adds an optional key field (NotRequired[str | None]) to the MultivariateFeatureOption TypedDict in the DynamoDB schema, representing a stable, human-readable identifier for multivariate feature variants. Additionally, an integration test has been added to verify that the key field is correctly validated and preserved. There are no review comments, so I have no feedback to provide.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 97.40%. Comparing base (819921a) to head (e765c57).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #242   +/-   ##
=======================================
  Coverage   97.39%   97.40%           
=======================================
  Files         104      104           
  Lines        4613     4619    +6     
=======================================
+ Hits         4493     4499    +6     
  Misses        120      120           

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