Fix when and unless return value

[netpok]

• Added or updated tests
• Documented user facing changes
• Updated CHANGELOG.md

Changes
This PR fixes the return value validation for the when and unless methods on the Models

I'm also not sure about this whole feature as it does what the when method should do but not what it actually can.

A more realistic (and invalid) version:

    /**
     * Apply the callback's query changes if the given "value" is true.
     *
     * @template TValue
     * @template TCallbackResult
     * @template TDefaultResult
     *
     * @param TValue  $value
     * @param callable($this, TValue): TCallbackResult $callback
     * @param callable($this, TValue): TDefaultResult|null  $default
     * @return TCallbackResult|TDefaultResult|$this
     */
    public function when($value, $callback, $default = null);

Or a valid version: A more realistic (and invalid) version:

    /**
     * Apply the callback's query changes if the given "value" is true.
     *
     * @param mixed  $value
     * @param callable($this, mixed): mixed $callback
     * @param callable($this, mixed): mixed $default
     * @return mixed|$this
     */
    public function when($value, $callback, $default = null);

Breaking changes
None

[szepeviktor]

Thank you for your contribution.

Could you add a test for each method (which fail without these modification)?

[netpok]

Updated existing test to fail

[netpok]

@szepeviktor Is this test acceptable?

[szepeviktor]

It seems so.

[canvural]

Thank you!