Use Type{T} self-types for rules on types on Julia 1.14

[devmotion]

Fixes #713.

On Julia 1.14, Core.Typeof(T) returns Core.TypeEgal{T} for a type T (JuliaLang/julia#62001). The rule macros use it for the function argument, so @non_differentiable/@scalar_rule/@opt_out rules on types became ambiguous with hand-written, more specific ::Type{T} rules. One example is @non_differentiable Array(::AbstractArray{Bool}) vs rrule(::Type{Array}, ::HermOrSym) in ChainRules.jl.

This PR replaces Core.Typeof with _selftype(@nospecialize(x)) = isa(x, Type) ? Type{x} : typeof(x). That is the definition of Core.Typeof up to Julia 1.13 and of Core.TypeEqOf, which lowering uses for method self-types on 1.14 (JuliaLang/julia#62287). Behaviour on Julia < 1.14 is unchanged.

I added a regression test to test/rule_definition_tools.jl. With ChainRulesCore 1.26.1 on 1.14.0-DEV.3529, the new test errors with MethodError: rrule(::Core.TypeEgal{NonDiffTypeExample}, ::Float64) is ambiguous. With this PR, the full test suite passes there.

On Julia 1.13.1 locally, one allocation test (projection.jl:462, ProjectTo(::Symmetric)) fails with 32 bytes allocated. It's unrelated to this change, and CI on main last ran before 1.13 was released.

Docs: I removed the "check which to use with Core.Typeof" tip from writing_good_rules.md, because on 1.14 it shows Core.TypeEgal{Bar}, the very signature that causes this issue. The @non_differentiable expansion example in rule_definition_tools.md now shows the defined self-type ::typeof(*).

Version bumped to 1.26.2.

🤖 Generated with Claude Code

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 93.39%. Comparing base (1a236fb) to head (1f7cf23).

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #714   +/-   ##
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  Coverage   93.38%   93.39%           
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  Files          15       15           
  Lines         998      999    +1     
=======================================
+ Hits          932      933    +1     
  Misses         66       66           

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