Add testing infrastructure for srsilo Rust tools

[gordonkoehn]

Problem

• No unit tests for critical data pipeline tools
• No integration tests for the srsilo role
• Code evolution risk without regression protection

Scope

1. Unit tests (low effort, high value):

   • URL construction functions
   • Timestamp parsing/validation
   • Sample deduplication logic

2. Refactor for testability (medium effort):

   • Extract pure functions from I/O
   • Add traits for HTTP client injection

3. Integration tests (higher effort):

   • Mock API server
   • End-to-end with test data in roles/srsilo/files/test_data/

Not in scope (for now)

• Full Ansible role integration test (commented out in CI)