Let a callable member of a union parameter take an argument it accepts

[SanderMuller]

Closes phpstan/phpstan#15414

With unresolvedTemplateArguments (bleedingEdge), $a->merge($b)->reject(isExcluded(...)) inferred Coll<Field|(Closure(Field): bool)> instead of Coll<Field>. Laravel's Collection::merge() and reject() have this shape.

observeLowerBound() splits a union parameter. When a member takes an argument, the argument does not also flow into a naked marker next to that member. For (callable(TValue): bool)|TValue, the callable member never records a lower bound, because its template is in a contravariant position. The loop therefore saw no change and treated the Closure as not taken, so the Closure became a lower bound of the TValue marker.

Now a callable member takes an argument that it accepts. This is the same rule as for a marker-free member, and as in UnionType::inferTemplateTypes(). A callable that the callable member does not accept, for example reject(isInt(...)) on Coll<Field>, still flows into TValue as on 2.3.0. TemplateArgumentObserver.cpp gets the same change, and the second commit bumps the expected turbo version.

Verification:

• tests/PHPStan/Analyser/nsrt/bug-15414.php fails without the change, with Coll<Field|(Closure(Field): bool)> on line 63, and passes with it.
• The third commit adds two cases where a member only maybe takes the argument: a raw \Closure, and Box<X> sent to Box<TValue>|TValue. Both behave as on the base. They fail under the two mutants that Infection reported on the first run, !isCallable()->no() and !isSuperTypeOf()->no().
• The full test suite passes without and with the turbo extension loaded: 22503 tests, 74 skipped.
• make phpstan reports no errors.
• Turbo: the strict build has no compiler warnings, smoke.php reports ALL OK, and signature-parity.php reports OK. The raw analysis output of src/Analyser/Generics and the new test file is identical with and without the extension. Without the .cpp change, the turbo build still infers the Closure, so the port is needed. I could not run make lint-turbo, because clang-tidy is not installed here.
• Performance: no change. I ran a cold bin/phpstan analyse src/Type src/Analyser with bleedingEdge and without turbo, 3 interleaved runs. The base took 7.09-8.05 s and 57.1-58.1 s user CPU, and this PR took 7.11-7.12 s and 57.6-58.4 s. The output is identical.

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