Let other config keys through the skip_was_running_backfill stub

The three seed migration specs stub Config#get so that only
:skip_was_running_backfill is allowed. After each example the shared
migration context runs every newer migration. Main now has
20260811120000_add_root_job_guid_to_jobs, which on Postgres calls
Config#get(:migration_psql_concurrent_statement_timeout_in_seconds).
The stub rejected that call, so the three specs failed on Postgres
only. The stub now calls the real method for every other key.

Author: joyvuu-dave

spec/migrations/20260601120100_seed_was_running_app_usage_events_spec.rb

@@ -150,6 +150,7 @@ def seed_app_event(suffix, state:, app_guid:)
 
     context 'when skip_was_running_backfill is set' do
       before do
+        allow_any_instance_of(VCAP::CloudController::Config).to receive(:get).and_call_original
         allow_any_instance_of(VCAP::CloudController::Config).to receive(:get).with(:skip_was_running_backfill).and_return(true)
       end
 

spec/migrations/20260601120200_seed_was_running_service_usage_events_spec.rb

@@ -105,6 +105,7 @@ def seed_service_event(suffix, state:, service_instance_guid:)
 
     context 'when skip_was_running_backfill is set' do
       before do
+        allow_any_instance_of(VCAP::CloudController::Config).to receive(:get).and_call_original
         allow_any_instance_of(VCAP::CloudController::Config).to receive(:get).with(:skip_was_running_backfill).and_return(true)
       end
 

spec/migrations/20260601120300_seed_was_running_task_usage_events_spec.rb

@@ -106,6 +106,7 @@ def seed_task_event(suffix, state:, task_guid:)
 
     context 'when skip_was_running_backfill is set' do
       before do
+        allow_any_instance_of(VCAP::CloudController::Config).to receive(:get).and_call_original
         allow_any_instance_of(VCAP::CloudController::Config).to receive(:get).with(:skip_was_running_backfill).and_return(true)
       end