fix: make feature flag disable_tenant idempotent to prevent 500 error

[CCimen]

Changes

• Changed set.remove() to set.discard() in FeatureFlag.disable_tenant() method
• Added regression tests for idempotent enable/disable operations

Why

When toggling templates OFF in the admin panel for a tenant that never had templates enabled, the system
returned a 500 Internal Server Error.

Root cause: set.remove(tenant_id) raises KeyError if the tenant_id doesn't exist in the set. Using
set.discard() is the idempotent alternative that safely handles this case.

This affects all users on the latest version who try to toggle templates when they were never enabled, or
toggle off twice in a row.

Testing

• Added test_disable_tenant_is_idempotent - verifies disabling a never-enabled tenant doesn't crash
• Added test_enable_tenant_is_idempotent - verifies enabling twice is safe

python -m pytest tests/unittests/feature_flag/test_feature_flag.py -v

Screenshots

N/A - Backend fix only