[mohamedbasuony]

Summary

Fixes #1898 by preserving Solang compiler diagnostics through the playground compile flow.

Compiler diagnostics were being reduced before reaching the UI: the backend kept only the substring after the first lowercase error: in compiler stderr, and the frontend displayed only one diagnostic field. This could hide warnings/context, multiline source spans, diagnostics without that exact marker, and useful fallback output.

What changed

• Preserve the full ANSI-stripped compiler stderr in the backend.
• Avoid replacing empty compiler stderr with a placeholder that would shadow useful fallback output.
• Return a small non-secret body for backend failures instead of an empty HTTP 500 response.
• Display compiler diagnostics in the frontend using this fallback order:
  compile_stderr > compile_stdout > stderr > stdout > generic fallback.
• Read non-200 response body text before falling back to statusText.
• Keep the API shape, compiler dependency, Docker image, and generated bindings unchanged.

Verification

Ran locally:

• git diff --check
• rustfmt --check crates/backend/src/services/compile.rs crates/backend/src/services/sandbox.rs
• cargo test -p backend
• cargo test
• npm run build --workspace=packages/frontend

Manual Docker-backed check:

• Posted an invalid contract to /compile and verified that the response preserves the compiler diagnostic text.
• Verified multiline/source-span formatting is preserved.
• Verified a valid compile control still succeeds.

npm run test --workspace=packages/app requires a running server at localhost:9000; without that server it fails with connection errors.

[salaheldinsoliman]

Could you rebase the PR to develop?
We can then merge this branch to develop and then to main.