Do not generalize an array union that is no larger than its members

[HawkForge]

For phpstan/phpstan#15459. Thanks @bmdevel for the report and the generator script.

When the branches of $r['o0'] ?? null merge back, the declared shape and its narrowed variant (o0 set and not null) are unioned. reduceArrays() merges them back into the declared shape, but processArrayTypes() then passes the result to optimizeConstantArrays(). That function counts the constant array values in the result, nested shapes included. In the report this is 197 + 20 × 3 = 257, which is above ConstantArrayTypeBuilder::ARRAY_COUNT_LIMIT, so the shape is generalized to non-empty-array<..., ...>&oversized-array. Every later read of a key on it then returns the union of all value types.

2.2 never got to optimizeConstantArrays() here. Its key numbering in processArrayTypes() overflowed at the 63rd distinct key, and the arrays then went through reduceArrays($arrayTypes, false) and returned early. 2.3 removed that numbering, so large shapes now reach the limit check.

With this change, a union that reduces to a single array is kept as it is when that array holds no more constant array values than the largest of the arrays it was made of. The limit is there to stop unions from growing, and a union like this one does not grow. Unions that do grow past the limit, and unions that stay as several arrays, are still optimized as before. The extra counting happens only when the single reduced array is above the limit. The same change is ported to turbo-ext/src/TypeCombinator.cpp, with the regenerated declarations header and the version bump in a separate commit.

tests/PHPStan/Analyser/nsrt/bug-15459.php is the issue's reproducer, with dumpType() replaced by assertType(). Without the fix it fails with (array{currency: string, price: float}&oversized-array)|string|null. It passes with the fix, both with and without the Turbo extension loaded. The full test suite, smoke.php, signature-parity.php and side-by-side.php pass.

Closes phpstan/phpstan#15459