Fix System.Text.Json version conflicts and missing FlinkDotNet.Common references in LearningCourse exercises

[Copilot]

Recent FlinkDotNet changes introduced version conflicts and missing assembly references that prevented LearningCourse exercises from building. This breaks the learning path for developers.

Changes

Package Version Alignment

• 31 exercise projects updated from System.Text.Json 9.0.0/8.0.5 → 9.0.7
• Resolves downgrade warnings: exercises now match FlinkDotNet.DataStream's dependency version
• Affected: Day01, Day03, Day04, Day07, Day10-Day13, Day15

Missing Assembly References

• 7 exercise projects now reference FlinkDotNet.Common.csproj
• Fixes CS0012 errors: Configuration type accessible without assembly not referenced errors
• Affected: Day03/Exercise34, Day04/Exercise42+44, Day13/Exercise131-134

Build Status

• LearningCourse/IntegrationTests.sln: ✅ 0 errors (131 warnings - pre-existing)
• All 15 days of exercises compile successfully

Example Fix

<!-- Before: Version conflict -->
<PackageReference Include="System.Text.Json" Version="9.0.0" />

<!-- After: Aligned with FlinkDotNet.DataStream -->
<PackageReference Include="System.Text.Json" Version="9.0.7" />

<!-- Added missing reference -->
<ProjectReference Include="..\..\..\..\FlinkDotNet\FlinkDotNet.Common\FlinkDotNet.Common.csproj" />

Original prompt

We changed a lot of functionalities which can break LearningCourse. Please run day by day tests in LearningCourse and fix any issue if failing. Please never run batch tests which will freeze the command as there are many tests.

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