Fix backend compile break after Jackson dependency upgrades

[Copilot]

The backend started failing at :compileJava on PR #494 after dependency updates because PropertyNamingStrategy.SNAKE_CASE is no longer available in the upgraded Jackson line. The failure occurred in descriptor JSON mappers used by index and onionperf parsing.

• Jackson API compatibility update

  • Replaced deprecated/removed PropertyNamingStrategy usage with PropertyNamingStrategies in the two failing Java classes.
  • Updated imports accordingly to match current Jackson APIs.

• Touched parser components

  • org.torproject.descriptor.index.IndexNode
  • org.torproject.descriptor.onionperf.ParsedOnionPerfAnalysis

import com.fasterxml.jackson.databind.PropertyNamingStrategies;

new ObjectMapper()
    .setPropertyNamingStrategy(PropertyNamingStrategies.SNAKE_CASE);

[Copilot]

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[JuliusHenke]

@copilot the backend tests still fail. Fix remaining issues.