fix: use FetchAs::Associative enum for fetchAll() in RlAnalyzer

[jjroelofs]

Summary

Fix the two pre-existing drupal-check (PHPStan) failures on 1.x by swapping the legacy \PDO::FETCH_ASSOC int constant for the new Drupal\Core\Database\Statement\FetchAs::Associative enum case.

What changed

Drupal 11 moved the fetch-mode argument of Connection::fetchAll() from the int-based PDO::FETCH_* constants to a typed FetchAs enum. Passing \PDO::FETCH_ASSOC still works at runtime (PDO defines it as an int), but PHPStan via mglaman/phpstan-drupal flags it as an argument.type error:

```
Parameter #1 $mode of method
Drupal\Core\Database\StatementInterface::fetchAll() expects
Drupal\Core\Database\Statement\FetchAs|null, int given.
```

Two call sites in `src/Service/RlAnalyzer.php` (lines 407 and 428) were the only remaining drupal-check failures on `1.x`. Both swapped to `FetchAs::Associative`, plus a matching `use Drupal\Core\Database\Statement\FetchAs;` import.

Why now

These two errors have been present on `1.x` since the repo adopted Drupal 11 compatibility. They were surfaced again during the PR #35 review as the only red CI check that was never blocking a merge but was always noise in the status rollup. Worth fixing in a tiny dedicated PR so every subsequent PR starts from a green baseline.

Verification

• `docker compose --profile lint run --rm drupal-check` → `[OK] No errors` (was: `Found 2 errors`)
• `docker compose --profile lint run --rm drupal-lint` → 0 errors, 0 warnings
• `docker compose --profile test run --rm e2e-test` → 88/88 assertions pass across all 6 test files
• PHP syntax check clean

Test plan

• CI runs all three checks (`drupal-lint`, `drupal-check`, `drush-e2e`) and all three pass
• No runtime regression in analytics endpoints that call `getArmsData()` or `getArmSnapshots()` (already covered by existing e2e tests hitting `rl:analytics:*` commands)

Scope guardrails

• Only touches two lines + one import in `RlAnalyzer.php`
• No schema changes, no API changes, no behaviour changes — `FetchAs::Associative` is the exact enum-equivalent of `\PDO::FETCH_ASSOC`
• Existing call sites in `getAllArmsData()` / `getTotalTurns()` / etc. in `ExperimentDataStorage` are unaffected (they already don't pass a fetch-mode argument, relying on Drupal's default `FetchAs::Object`)