Validate geocoding search query input (#527)

[maltehuebner]

Addresses the input-validation half of #527.

Changes

• TypeaheadController::searchAction now trims the query parameter and validates its length (2–128 chars). Missing/empty/over-long input returns HTTP 400 with a JSON error instead of the previous HTTP 500 (GeocodeQuery::create(null) → TypeError, since Geocoder::query() is typed string).
• Non-scalar array input (?query[]=x) is already rejected as 400 by Symfony's InputBag::get(), so no extra handling is needed.

Deliberately not implemented (deferred)

• Rate limiting (symfony/rate-limiter). This requires adding the dependency plus a framework.rate_limiter config block and a boot-critical cache pool, and its behaviour can only be meaningfully verified against a running app with Redis. The container could not be booted in the review environment (the local vendor/ is missing symfony/runtime, so bin/console and WebTestCase cannot start), so I could not verify that the wiring compiles or that limiting actually triggers. Rather than commit unverifiable boot-critical config, this is left for a follow-up. Suggested config for the follow-up:

  # config/packages/rate_limiter.yaml
  framework:
      rate_limiter:
          search:
              policy: 'sliding_window'
              limit: 30
              interval: '1 minute'

  then inject RateLimiterFactoryInterface $searchLimiter and consume by client IP in searchAction, returning 429 when the limit is exceeded.
• Route requirements. query is a query-string parameter, not a path segment, so route requirements do not apply.

Verification

• vendor/bin/phpunit --testsuite="Project Test Suite" — OK (121 tests)
• vendor/bin/phpstan analyse --no-progress — No errors
• A controller/integration test for the 400 path needs a bootable kernel (blocked here; tracked by [Mittel] Testsuite: Duplikate, falsche Namespaces, Copy-Paste-Fehler, Integration-Suite läuft nie in CI, große Abdeckungslücken #539's CI integration-suite work).

Closes #527

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