fix: trim the app environment and skip blank values

[catinspace-au]

Brings get_app_env() in scalo-py to parity with scalo-rs, which already trims each candidate and treats a blank value as unset.

• get_app_env() now strips whitespace from APP_ENV, ENVIRONMENT and ENV in turn, and falls through to the next variable when a value is empty or whitespace-only, before defaulting to "development".
• The duplicated inline expression in get_standard_env_vars() now calls get_app_env() instead of repeating the resolution logic.
• Added parity tests: leading/trailing whitespace is trimmed, a blank or whitespace-only value falls through, and all-blank still defaults to "development".
• Updated the module docstring and docs/core-pillars/CONFIG.md to note the trimming and blank-as-unset behaviour.

Without this, APP_ENV=" production" (a padded value, e.g. from a shell export or a K8s manifest) resolves to production in scalo-rs but development in scalo-py.