[Platform] Add InMemoryPlatform and InMemoryRawResult for testing

[RamyHakam]

Q	A
Bug fix?	no
New feature?	yes
Docs?	yes
Issues	none
License	MIT

This PR adds two test-friendly classes to the Symfony AI Platform component:

InMemoryPlatform

The InMemoryPlatform is a test-specific implementation of PlatformInterface that returns a TextResult wrapped in a ResultPromise.
It supports:

• A fixed string response (e.g. 'Fake reply')
• A callable response based on the Model, input, and options

This makes it ideal for unit or integration tests, or even functional tests where a full AI stack is not needed.

InMemoryRawResult

The InMemoryRawResult implements RawResultInterface and is used internally by InMemoryPlatform. It wraps:

• A structured array (getData())
• A generic object (getObject())

to simulate what real raw results look like in production usage.

The Problem

While testing new code that uses the AI platform, developers often need to manually create PlatformInterface instances. The existing Platform and PlatformFactory classes are tightly coupled to real AI providers.
This in-memory implementation simplifies unit and integration testing by allowing developers to inject a dummy platform and return mock results without any external API calls.

Example

$platform = new InMemoryPlatform('Test reply');
$response = $platform->invoke(new Model('test'), 'Hello');
echo $response->asText(); // "Test reply"

[chr-hertel]

let's use constructor property promotion here - and I'm not sure if a default value makes sense here 🤔

isn't the usual case that you want to provide something explicitly?

[RamyHakam]

@chr-hertel I already tried that, but property promotion does not allow callable
PHP will throw Fatal error: Property cannot have type 'callable'

And regarding the default value, you are right, it's a valid point, I will remove it

[chr-hertel]

how about using Closure instead?

[RamyHakam]

@chr-hertel Done , Thaks

[chr-hertel]

Thank you @RamyHakam.