fix(debugging): stop debugger displays rendering as a bare null

[glennawatson]

What kind of change does this PR introduce?

Bug fix.

What is the new behavior?

A debugger display always shows that an object is there, whatever its members hold.

• ApiResponse<T> leads with the type and the status code. Hovering a response that came back without content now reads ApiResponse: NotFound, Content = null instead of a bare null.
• Ten other shipped types carried the same shape and are fixed with it - ValidationApiException, StubApiResponse<T>, RefitSettings, BodyAttribute, MultipartItem, PropertyAttribute, RestMethodParameterInfo, SettingsFor<T>, RouteMatcher and XmlContentSerializerSettings. Each keeps the member it showed and gains the literal text that makes a null value legible.
• DebuggerDisplayTests keeps it that way. It walks the shipped assemblies and fails any display whose sole placeholder can be null, and any placeholder naming a member that does not exist. The offender list in this PR is what that test reported, not a hand-picked set.

What is the current behavior?

A display made of nothing but one member renders as a bare null when that member is null.

• ApiResponse<T> carried [DebuggerDisplay("{Content}")], so a response with no content - a request error, or a 204 - showed as null in the debugger. It reads as though the response object itself does not exist, and sends the reader looking for a NullReferenceException that was never thrown.

Closes #2313

What might this PR break?

None at runtime.

• Debugger displays have no effect outside a debugging session.
• The public API baselines record [DebuggerDisplay], so the format-string changes show up as baseline diffs across every target. That is the analyzer recording an attribute argument, not a change to any callable surface.

Checklist

• I have read the Contribute guide
• Tests have been added or updated (for bug fixes / features)
• Docs have been added or updated (for bug fixes / features)
• Changes target the main branch
• PR title follows Conventional Commits

Additional information

The reporter suggested prefixing the display, for example ApiResponse:{Content}. This goes slightly further for the response and exception types by also showing the status code, which is the state worth seeing precisely when the content is missing.

{ToString(),nq} displays are left alone - those types override ToString(), and that is the idiomatic form rather than the shape that caused the report.

Hand-authored file worth reviewing: DebuggerDisplayTests.cs. Everything else is a one-line attribute change or regenerated baseline output.

[sonarqubecloud]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
0.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarQube Cloud

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.91%. Comparing base (1643323) to head (1bd3f50).

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2315   +/-   ##
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  Coverage   99.91%   99.91%           
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  Files         194      194           
  Lines       10138    10138           
  Branches     1947     1947           
=======================================
  Hits        10129    10129           
  Partials        9        9           

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[github-actions]

This pull request has been automatically locked since there has not been any recent activity after it was closed. Please open a new issue for related bugs.