[Coverage] BuildBot.Json — increase code coverage to 100%

[credfeto]

Problem

BuildBot.Json has 0% line coverage and 0% branch coverage. There are no tests for any of the types in this assembly.

Closes part of #356.

Current State

• Line coverage: 0%
• Branch coverage: 0%

Files Requiring Tests

All source files in src/BuildBot.Json/:

• AppSerializationContext.cs
• PongDto.cs
• SerializationContext.cs

Acceptance Criteria

• A new BuildBot.Json.Tests project exists.
• All source types in BuildBot.Json are covered at 100% line and branch coverage.
• All tests pass in CI.