[AArnott]

Builds on #4192 (merged), which makes the C# 15 test project default to preview. This PR finishes union (SyntaxKindEx.UnionDeclaration) support in the rules that still skipped unions.

Closes #4182. Once this merges, every sub-issue of #4180 is closed, so #4180 can be closed too.

Approach

UnionDeclarationSyntax is a TypeDeclarationSyntax, so I added SyntaxKindEx.UnionDeclaration to the shared SyntaxKinds.BaseTypeDeclaration and SyntaxKinds.TypeDeclaration lists, the same way records were added. Each of these is still a syntax-kind registration, so code without unions does no extra work.

This differs from #4191, which added a separate union registration to each rule. With the shared lists, those per-rule lines in SA1201, SA1202, SA1203, SA1400, SA1600 and SA1601 would run each rule twice per union, so I removed them. Their existing union tests still pass, each with exactly one diagnostic. If you'd rather keep per-rule registrations, I can switch to that. It would mean about 25 extra registration lines.

Rules that now analyze unions

Area	Rules	Via
Layout	SA1500, SA1502, SA1505, SA1508, SA1516	shared lists
Layout	SA1506, SA1514	their own kind lists
Ordering	SA1205	shared list
Ordering	SA1207, SA1214	their own kind lists
Readability (the union's case type list)	SA1106, SA1110, SA1111, SA1112, SA1113, SA1114, SA1115, SA1116, SA1117	shared lists
Readability	SA1137	shared list, plus the nested-type attribute check
Ordering	SA1200	A union in the global namespace now suppresses SA1200 the same way other types do
Documentation	SA1604–SA1608, SA1612–SA1614, SA1616, SA1625, SA1629	ElementDocumentationBase / summary base classes
Documentation	SA1618, SA1619, SA1620–SA1622	shared list
Documentation	SA1648	its own kind list

Requested rules not in the table: SA1201–SA1204, SA1206, SA1300, SA1400, SA1402, SA1600, SA1601 and SA1649 were already done in #4188/#4191/#4096.

Adjustments

• SA1611 skips unions. A union's parameter list declares its case types (union Pet(Cat, Dog)), which have no names to document. Without this, the shared list would have reported each case type with an empty name. TestUnionCaseTypesAsync covers it.
• Code fixes: SA1205 (adds the access modifier through UnionDeclarationSyntaxWrapper) and SA1502 (expands a single-line union body).

SA1642 union constructors (5032740, 4ca6e40)

SA1642 and its code fix treated union constructors as class constructors. So on master:

• SA1642 required Initializes a new instance of the <see cref="Pet"/> class. on union constructors.
• The code fix inserted the class text.
• SA1642SA1643CodeFixProvider.GetTypeParameterList had no union case. In Debug builds it hit Debug.Assert ("Unhandled type UnionDeclaration"). In Release builds the cref for a generic union lost its type parameters (Result instead of Result{T}).

5032740 fixes the type parameter list. 4ca6e40 adds union wording, following the existing class/struct resources: a new TypeTextUnion string in every DocumentationResources culture. Union constructors now expect, and the code fix inserts:

• Initializes a new instance of the <see cref="Result{T}"/> union.
• Initializes static members of the <see cref="Pet"/> union.
• The private-constructor form ("Prevents a default instance of the ... union from being created.") comes from the same templates.

The struct wording on a union constructor is now reported. documentation/SA1642.md has a union example. SA1643 (destructors) has no type-kind text, and unions can't have destructors, so it needed no change.

The translations are best effort: de "Union", fr/es/nl/en-GB "union", pt-BR "união", cs "sjednocení", pl "unii", ru "объединения". The French template is de la {0}, so it produces "de la union" rather than "de l'union"; the Spanish template gives "de la union". Fixing the article would mean changing the shared template, which I left alone.

Tests

There are new C# 15 tests for SA1106, SA1110, SA1111, SA1113–SA1117, SA1137 (2), SA1200, SA1205, SA1207, SA1214, SA1500, SA1502, SA1505, SA1506, SA1508, SA1514, SA1516, SA1604, SA1606, SA1611, SA1618, SA1620, SA1642 (5) and SA1648.

• Each expects exactly one diagnostic per violation.
• Each rule that has a code fix gets a code-fix test.
• They rely on Default the C# 15 test project to the preview language version #4192's preview default instead of passing LanguageVersion.Preview.
• Negative check: with the product changes reverted, every new test fails except the SA1611 one, which guards against the shared-list change.
• Not separately tested, but covered by the shared lists: SA1112, SA1605, SA1607, SA1608, SA1612–SA1614, SA1616, SA1619, SA1621, SA1622, SA1625, SA1629.

Local run (C# 15, net6.0, Linux): 9,593 tests. The only new failures are the 5 new code-fix tests that show the known Linux CRLF difference; Windows CI is authoritative for those.

[codecov]

Codecov Report

❌ Patch coverage is 99.48187% with 4 lines in your changes missing coverage. Please review.
✅ Project coverage is 97.54%. Comparing base (a19eede) to head (4ca6e40).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #4193      +/-   ##
==========================================
+ Coverage   97.53%   97.54%   +0.01%     
==========================================
  Files        1250     1277      +27     
  Lines      133401   134157     +756     
  Branches     4290     4291       +1     
==========================================
+ Hits       130109   130863     +754     
- Misses       2150     2152       +2     
  Partials     1142     1142              

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