ci: use DIESEL_CONFIG_FILE=/dev/null to suppress schema regeneration

Replacing the previous git checkout approach: using DIESEL_CONFIG_FILE
  pointing to /dev/null gives diesel an empty config with no [print_schema]
  section, so schema.rs is never written. This is cleaner than restoring
  the file after the fact, which would silently hide genuine schema drift.

.github/workflows/integration-test.yml

@@ -87,13 +87,12 @@ jobs:
 
       # Verify that all PostgreSQL migrations apply cleanly to a fresh database.
       # This catches broken SQL before it reaches a production deployment.
-      # Restore src/db/schema.rs after running: diesel.toml has [print_schema]
-      # configured, so diesel rewrites the file with backend-specific types after
-      # migration. The checked-in schema.rs must be restored before compiling.
+      # DIESEL_CONFIG_FILE=/dev/null disables the [print_schema] setting from
+      # diesel.toml so that schema.rs is not overwritten during the check.
       - name: Run PostgreSQL migrations
-        run: |
-          diesel migration run --migration-dir migrations/postgresql
-          git checkout -- src/db/schema.rs
+        env:
+          DIESEL_CONFIG_FILE: /dev/null
+        run: diesel migration run --migration-dir migrations/postgresql
 
       # Run the test suite with a live PostgreSQL instance available.
       # Existing tests are unit tests (no DB access), but DATABASE_URL is set
@@ -157,11 +156,11 @@ jobs:
         run: cargo install diesel_cli --no-default-features --features mysql
 
       # Verify that all MySQL migrations apply cleanly to a fresh database.
-      # Restore src/db/schema.rs for the same reason as the PostgreSQL job above.
+      # DIESEL_CONFIG_FILE=/dev/null for the same reason as the PostgreSQL job.
       - name: Run MySQL migrations
-        run: |
-          diesel migration run --migration-dir migrations/mysql
-          git checkout -- src/db/schema.rs
+        env:
+          DIESEL_CONFIG_FILE: /dev/null
+        run: diesel migration run --migration-dir migrations/mysql
 
       # Run the test suite with a live MySQL instance available.
       - name: Run tests (mysql)