dev: enable nullable where no errors, treat warnings as errors

[marksvc]

Files with #nullable enable will have an error from new code like
string foo = null. (Instead of string? foo = null.)

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This change is 

[devin-ai-integration]

✅ Devin Review: No Issues Found

Devin Review analyzed this PR and found no potential bugs to report.

View in Devin Review to see 2 additional findings.

[marksvc]

This takes a step toward our C# code enforcing non-nullable type usage.

[RaymondLuong3]

Why not just enable nullable from the .csproj files?
<nullable>enable</nullable>

@RaymondLuong3 reviewed all commit messages and made 1 comment.
Reviewable status: 0 of 255 files reviewed, all discussions resolved.

[marksvc]

If we enable nullable from the csproj files now, it would apply that setting to all C# files in the project. We would get many errors where things like string foo = null are done.
This PR just sets 'nullable enable' on files that won't produce such errors. For files that do not have #nullable enable, when we work on those, we can one at a time change string foo = null to string? foo = null, and set #nullable enable on that file.
Once all files can have 'nullable enable' without errors, then we can set <Nullable>enable</Nullable> project wide in the csproj files and remove the #nullable per-file settings.

[Nateowami]

@marksvc Is it possible to enable it globally, and then selectively, at a file level, disable it, so all new code will automatically have it enabled?

[marksvc]

@Nateowami Thank you for suggesting that. There are around 250 files that can have nullable enabled, and about 161 files that can not (without further work). Now the csproj files enable nullable, which will desirably apply to new files automatically, and not-yet-migrated/fixed-up files are marked with #nullable disable warnings.

When we change a file so it conforms with non-nullable types, we can remove its #nullable disable warnings line.

In file src/SIL.XForge.Scripture/Pages/Shared/_SelectLanguagePartial.cshtml I was not successful applying #nullable disable warnings. Instead, I modify the code so it does not show the <select> element if requestCulture != null. My understanding is that the asp-for is both an input and output binding. It would be read from to show the current language in the element, and written to when a language is selected. Neither of those are going to work if requestCulture is null, and AI suggests the page would show an error in that case. With the change, it will not try to read to or write from requestCulture if it is null, and will just omit the language chooser from the page in that case.

@marksvc made 1 comment.
Reviewable status: 0 of 255 files reviewed, all discussions resolved (waiting on RaymondLuong3).

[devin-ai-integration]

Devin Review found 1 new potential issue.

View 1 additional finding in Devin Review.

[devin-ai-integration]

🚩 Inconsistent null handling between _Footer.cshtml and _SelectLanguagePartial.cshtml

Both _Footer.cshtml:12 and _SelectLanguagePartial.cshtml:11 obtain requestCulture from Context.Features.Get<IRequestCultureFeature>(), which can return null. The PR properly adds a null check in _SelectLanguagePartial.cshtml:31 (@if (requestCulture != null)), but in _Footer.cshtml:11 it only adds #nullable disable warnings to suppress the compiler warning. If requestCulture is null, _Footer.cshtml:14 would throw a NullReferenceException when accessing requestCulture.RequestCulture.UICulture.Name. While IRequestCultureFeature should always be present when UseRequestLocalization middleware is configured (src/SIL.XForge.Scripture/Startup.cs:252), this is a pre-existing inconsistency that the PR's nullable migration makes more visible. The AGENTS.md rule 'Corner-cases happen. They should be handled in code' suggests the _Footer.cshtml should also get a proper null check rather than just warning suppression.

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[marksvc]

Fixing potential NREs is for possible followup, and not intended to be part of this PR. It was done in _SelectLanguagePartial.cshtml where I did not seem to be able to just #nullable disable warnings.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 81.31%. Comparing base (e8ae5b7) to head (1eef167).
⚠️ Report is 3 commits behind head on master.
✅ All tests successful. No failed tests found.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #3750   +/-   ##
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  Coverage   81.31%   81.31%           
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  Files         620      620           
  Lines       39079    39079           
  Branches     6376     6376           
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  Hits        31778    31778           
  Misses       6329     6329           
  Partials      972      972           

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[Nateowami]

Changes look good, except that I can't build locally due to warnings being treated as errors, and these two errors being present:

src/SIL.XForge.Scripture/Controllers/OnboardingRequestRpcController.cs(490,41): error CS1998: This async method lacks 'await' operators and will run synchronously. Consider using the 'await' operator to await non-blocking API calls, or 'await Task.Run(...)' to do CPU-bound work on a background thread. [/home/nate/src/web-xforge/src/SIL.XForge.Scripture/SIL.XForge.Scripture.csproj]
src/SIL.XForge.Scripture/Controllers/OnboardingRequestRpcController.cs(544,41): error CS1998: This async method lacks 'await' operators and will run synchronously. Consider using the 'await' operator to await non-blocking API calls, or 'await Task.Run(...)' to do CPU-bound work on a background thread. [/home/nate/src/web-xforge/src/SIL.XForge.Scripture/SIL.XForge.Scripture.csproj]

@Nateowami reviewed 418 files and all commit messages, made 1 comment, and resolved 1 discussion.
Reviewable status: complete! all files reviewed, all discussions resolved (waiting on marksvc).