…ldException in VS (#6158)

* fix(analyzers): decouple code fixers from Rules to prevent MissingFieldException in VS (#6157)

Code fixer assemblies ship in the version-agnostic analyzers/dotnet/cs
folder and resolve their analyzer-assembly dependency at runtime by
simple name. Visual Studio cannot unload analyzer assemblies, so after
a package update (or with mixed TUnit versions in one session) the new
code fixers can bind against a stale TUnit.Analyzers.dll. The eagerly
evaluated FixableDiagnosticIds then JITs a field reference to
Rules.MSTestMigration / Rules.NUnitMigration, which the stale assembly
does not contain, throwing MissingFieldException on every lightbulb
pass.

Fix: introduce DiagnosticIds const classes (compile-time baked into
consuming IL) and remove every Rules reference from both code fixer
assemblies. Add IL-level regression tests asserting the built code
fixer dlls contain no TypeReference to Rules.

* refactor: consolidate IL decoupling guard, cover AspNetCore codefixer, address review

- Extract duplicated PEReader scan into SharedTestHelpers/RulesDecouplingVerifier
  (linked into all three codefixer test projects)
- Cover the third vulnerable codefixer family: TUnit.AspNetCore.Analyzers gets
  DiagnosticIds consts + IL guard test (same #6157 pattern, was missed)
- Verifier also flags DiagnosticIds type refs so const -> static readonly drift
  fails the test (review feedback)
- Add missing readonly on InstanceTestMethod / MatrixDataSourceAttributeRequired
- Collapse the repeated rationale doc comment to one canonical home

* docs: note pre-existing TUnit0062 ID collision with TUnit.Analyzers

Surfaced by review on #6158; renumbering is user-facing and tracked
separately.

* Fix indexer interface mock wrappers

* fix(mocks): generate generic call wrappers

* test(mocks): cover typed returns arities

* fix(mocks): guard indexer return metadata

* fix(mocks): keep verification helper internal