Fix cross-platform (non-Windows) test failures in process agent, artifact URIs, code grader, LSP path test

[devin-ai-integration]

Summary

Full suite on Linux: 2239 → 2237 passing / 2 failing. Seven of the nine failures were real non-Windows bugs, not test noise:

• ProcessAgent: StandardInput.Close() sat outside the try { WriteLineAsync } catch (IOException), so a child that exits before reading its task JSON broke the pipe twice — the write was caught, the close wasn't. Moved the close inside the try.
• LocalArtifactStore.GetAccessUrl: new Uri($"file:///{fullPath}") yields file://tmp/x on Unix (the leading / of /tmp collapses into the authority slashes). Now new Uri(fullPath, UriKind.Absolute).
• CodeGrader: on Unix the shell script was passed as a flat Arguments = $"-c {expandedCommand}", which whitespace-splits into sh -c grep -q ... file so sh only ever saw grep. Now uses ArgumentList.Add("-c"); ArgumentList.Add(expandedCommand); Windows keeps cmd /c <string>.
• LspClientTests.PathToUri_Converts_Windows_Path: the inputs are drive-letter paths and PathToUri calls Path.GetFullPath, which only treats C:\... as rooted on Windows. Guarded the theory with if (!OperatingSystem.IsWindows()) return;.

All four bugs also exist on ramseur/sovrant development (verified by running those projects in a clean worktree of upstream), so this isn't merge fallout.

Still failing (not fixed here — product decision)

ModelOverrideLoaderTests.LoadAll_RegistersBundledOverrides / LoadAll_OllamaGlob_SetsWorkaround. The gemma-4 entries in src/Sovrant.Api/Capabilities/model-overrides.json carry "expires": "2026-07-01", and LoadFromStream correctly skips expired entries — so as of today the bundled overrides no longer register and the tests fall back to CapabilitySource.Default. The expiry mechanism is working as designed; the fix is either to extend/remove the expiry or to retire the workaround and the tests with it. Left alone rather than guessing.

Link to Devin session: https://app.devin.ai/sessions/d1b1be50c04f4787bcc00befb37bf805
Open in Devin Desktop: https://app.devin.ai/desktop/session/d1b1be50c04f4787bcc00befb37bf805?variant=devin
Requested by: @xingh

[devin-ai-integration]

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