Compute the verbosity level of an array shape key only when reporting an error - #6651

… an error

`ConstantArrayType::checkOurKeys()` (used by `ConstantArrayType::accepts()`) calls `VerbosityLevel::getRecommendedLevelByType($valueType, $otherValueType)` for every key it checks. That call walks both value types completely, but its result is only used to describe the two types in the error message of a key whose value is rejected. For every accepted key the walk is wasted, and when a key's value is itself an array shape, the walk covers that whole nested shape too.

This PR moves the computation so it only runs when a message is actually built. In PHP it becomes a closure called from the two places that build the message. The C++ twin in turbo-ext (`turbo-ext/src/ConstantArrayType.cpp`) gets the same treatment, with the call moved into `offsetReason()`. Error messages are unchanged.

I added a phpbench scenario, `tests/bench/data/nested-array-shape-accepts.php`, with a 150-key shape where 40 keys refer to a nested 150-key shape, passed to a parameter of the same shape.

Results (wall time, 3 alternating runs after a discarded warm-up):

| Benchmark | Before | After |
|---|---|---|
| phpbench `nested-array-shape-accepts.php` | 231.2-231.7 ms | 207.6-208.8 ms |
| [reproducer](https://github.com/Kocal/sf-ux-css-phpstan-reproducer) app, `phpstan-nested.neon`, without turbo extension | 23.03-23.25 s | 18.80-19.09 s |
| same app, with turbo extension loaded | 8.16-8.24 s | 7.12-7.15 s |

The reproducer is a small Symfony app that analyses, at level 6, one method taking the symfony/ux-css `CssStyles` shape (847 keys, 144 of them referring to a nested copy of the same shape) and passing it to one `css()` call. Same errors before and after (none).

The rest of the phpbench suite stays within its thresholds, the largest slowdown being +1.3% (bug-14869.php, 259 ms vs 256 ms). `tests/PHPStan/Rules` and `tests/PHPStan/Type` pass with and without the turbo extension loaded, and `turbo-ext/tests/smoke.php` plus `turbo-ext/tests/signature-parity.php` pass too.

This is independent from phpstan#6643 and phpstan#6645, which fix other redundant walks over the same kind of shape.

```
XDEBUG_MODE=off tests/vendor/bin/phpbench run --variant=nested-array-shape-accepts.php --report=aggregate
```