Test suite: remove duplicate, fix namespaces/typos, add missing unit tests (#539)

[maltehuebner]

Addresses the verifiable, unit-level items of #539.

Changes

Structure & quality

• Removed the duplicated PollutantViewModelTest at the stale path tests/Pollutant/ViewModel/ (there is no matching src/Pollutant/; the tests/Air/... copy mirrors src/Air/ViewModel/). Its one unique case, testPollutionLevel, was ported into the kept file so coverage is preserved.
• Fixed the PSR-4 violation in tests/Controller/ControllerTest.php (declared namespace App\Tests\Controller\Api; while living in tests/Controller/) → App\Tests\Controller.
• Removed the $client = $client = static::createClient(); copy-paste typos (3× in ControllerTest, 1× in ApiControllerTest).
• Assertions: assertSame for identity/value equality, and assertEqualsWithDelta(..., 0.01) for the computed float in DistanceCalculatorTest (previously relied on the implementation's round()).

New unit tests (previously 0 coverage)

• ValueDataConverterTest — the pollutant-identifier mapping edge cases from the issue: known identifier, case-insensitivity, underscore stripping (UV_INDEX_MAX → uvindexmax), and unknown → null (via reflection on the protected static convertPollutant).
• UVIndexMaxTest and UVIndexMaxLevelTest — the UVIndexMax / UVIndexMaxLevel gap explicitly called out in the issue.

Deliberately not implemented / deferred

• Activating the Integration suite in CI. Requires PostGIS + Redis service containers, a DB schema (explicitly depends on the migrations issue) and a bootable kernel. The container cannot be booted in this environment (local vendor/ lacks symfony/runtime, so WebTestCase cannot start), so this cannot be built or verified here.
• Decoupling testImpressPage from the network (config/packages/impress.yaml uses source: remote_json). A test-env override is a config change that needs a kernel boot to verify; deferred with the CI-integration work.
• EntityMerger tests. Its three confirmed bugs are tracked in [Hoch] Mass Assignment im EntityMerger + wirkungsloser #[Ignore]-Check #523 and being fixed separately; writing tests that assert the corrected behaviour here would either duplicate that work or turn the suite red against current main. Left to accompany the [Hoch] Mass Assignment im EntityMerger + wirkungsloser #[Ignore]-Check #523 fix.
• RequestConverter / deserializeRequestBodyToArray / StationCache tests. These need a geocoder mock/kernel/DB or touch code covered by the 500-route/[Hoch] Mass Assignment im EntityMerger + wirkungsloser #[Ignore]-Check #523 issues; deferred.

Verification

• vendor/bin/phpunit --testsuite="Project Test Suite" — OK (129 tests, was 121)
• vendor/bin/phpstan analyse --no-progress — No errors
• The Controller-test edits (namespace/typo/assertSame) are static and safe but live in the Integration suite, which is not runnable locally here.

Refs #539 (partial)

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