[13.x] Improve types of InteractsWithData when* helpers

[dfinchenko]

The InteractsWithData::whenEnum() method already carries full generic type information for its callback/default and return type, so static analysis can infer the precise result of a whenEnum(...) call. Its sibling conditional helpers in the same trait - whenHas(), whenFilled() and whenMissing() - share the identical shape (run $callback($value) when the condition holds, otherwise run $default() or return $this) but are still typed @return $this|mixed, so a call collapses to mixed.

This PR brings those three helpers in line by giving them the same generics:

@template TReturn
@template TReturnDefault = never

@param  string  $key
@param  callable(mixed): TReturn  $callback
@param  (callable(): TReturnDefault)|null  $default
@return $this|TReturn|TReturnDefault

The callback receives the resolved value (data_get($this->all(), $key)), which is mixed, hence callable(mixed): TReturn.

assertType coverage for all three helpers is added under types/. No behavior changes.