Incorrect `reason` used for environment default flags

[khvn26]

Currently the engine emits DEFAULT whenever the evaluation result is the environment default flag.

This should be:

1. STATIC if there are no targeting rules configured for the feature
2. DEFAULT if there are targeting rules configured, but the evaluation doesn't match them

i.e. if there are segment overrides for a feature, but they don't match the identity we are evaluating for, then return DEFAULT. If there are no segment overrides, return STATIC.

See Resolution Reason.

Acceptance Criteria:

• Change implemented in this repository and tested
• New version of engine released
• New engine version integrated into Flagsmith/flagsmith, Flagsmith/edge-api, Flagsmith/flagsmith-python-client, Flagsmith/edge-proxy
• Update reason logic in Flagsmith/flagsmith-go-client

[matthewelwell]

Updated the description based on a conversation with @aepfli .