Incorrect `reason` used for environment default flags

[khvn26]

Currently the engine emits DEFAULT for environment default flags. We should use STATIC instead.

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