Upgrade to .NET 9 and refactor project structure

[mak-thevar]

Summary

• move the library into src/ and tests into tests/, upgrade both projects to net9.0, refresh NuGet dependencies, and pin the SDK with a new global.json
• harden image processing by disposing streams deterministically, returning valid output streams, and enriching guard clauses across the service and compressor implementations
• update the S3 uploader to support injected clients and async disposal while modernizing the response model and test harnesses to reflect the new structure

Testing

• dotnet test (fails: dotnet is not available in the execution environment)

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https://chatgpt.com/codex/tasks/task_e_68d3d40ce288832f893abe61a268133a