Add comprehensive tests for timezone handling in station board queries

[Copilot]

Implements test coverage for the timezone logic introduced in commit 2d93bbc, which handles German time (Europe/Berlin) for date parameters in station board and train connection queries.

Test coverage added:

• User-provided dates: Validates dates are used directly without conversion (German time assumed)
• Auto-generated dates: Confirms UTC→Europe/Berlin conversion when no date provided
• Fallback behavior: Tests DateTime.Now fallback when timezone lookup fails
• Consistency across calls: Verifies timezone logic remains consistent with mixed call patterns

Example test:

[Fact]
public async Task GetStationBoardAsync_WithProvidedDate_UsesDateDirectlyWithoutTimezoneConversion()
{
    var germanTime = new DateTime(2025, 11, 6, 14, 30, 0);
    await service.GetStationBoardAsync(evaNo, germanTime);
    
    // Verifies path contains exact provided date without conversion
    Assert.Contains("plan/8000105/251106/14", capturedPath);
}

All 46 tests passing (5 new timezone tests added).

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