Incorrect `reason` used for environment default flags

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](https://openfeature.dev/specification/types/#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