feat(router): add load balancing support and unit tests

[gianpierofasulo]

Summary

This PR introduces load balancing capabilities to the Router module, allowing requests and tasks to be distributed across available handlers, along with a dedicated test suite.

Changes Made

• Load Balancing Logic: Implemented routing distribution logic in src/Router/.
• Test Coverage: Added unit tests in tests/Router/ to validate distribution scenarios and fallback behavior.

Motivation

Adding built-in load balancing prevents bottlenecking on single handlers, improves system resilience, and ensures balanced throughput.

How Has This Been Tested?

• Unit tests added under tests/Router/.
• Test suite executed locally with all checks passing.

[ilvalerione]

This PR introduces a new sub directory src/Router that marks all the files as new. RouterProvider already exists directly under src. You should revert the repo to the original package structure.

[gianpierofasulo]

Thanks for the review! I have restored the original directory structure and aligned all namespaces with the project's PSR-4 configuration (NeuronAI\ -> src/, NeuronAI\Tests\ -> tests/):

• Kept RouterProvider.php directly under src/ with namespace NeuronAI.
• Placed routing rules under src/Rules/ with namespace NeuronAI\Rules.
• Placed test files directly under tests/ with namespace NeuronAI\Tests.
• Preserved existing fallback mechanisms while integrating the load balancing enhancements.

[ilvalerione]

Inspired by this PR I decided to take a different approach.

The RoundRobinRule is now deprecated, and I implemented a new LoadBalancingRule that equally distribute the requests against the configured providers by default, basically working as a round-robin. Then you can specify different distribuition weights: https://github.com/neuron-core/router#loadbalancingrule

I also took the chance for this release to introduce a new ability in the fallback strategy to retry with a backoff time.

It's available in the 1.x branch, compatible with Neuron 3.x, and it's already ported in the 2.x branch compatible with the new Neuron 4.x: https://github.com/neuron-core/router#retry-with-backoff

Thank you for your contribution.