Allow external Prism providers in PrismGateway

[meirdick]

Summary

• toPrismProvider() now returns the driver name as a string for unrecognized drivers instead of throwing InvalidArgumentException
• Return type changed from PrismProvider to PrismProvider|string (matches what Prism::using() already accepts)
• All existing built-in drivers continue to map to their PrismProvider enum values — zero behavior change for current users

Problem

External Prism providers registered via PrismManager::extend() — like prism-php/bedrock (100k+ Packagist downloads) — cannot be used through the agent() system because toPrismProvider() throws for any driver not in its hardcoded match.

The Prism layer already supports this: Prism::using() accepts string|ProviderEnum, and PrismManager::resolve() checks customCreators first. The gap is only in the Laravel AI gateway layer.

Fix

- default => throw new InvalidArgumentException('Gateway does not support provider ['.$provider.'].'),
+ default => $provider->driver(),

The string driver name flows through to Prism::using() → PrismManager::resolve() which resolves custom providers via extend(), or throws its own InvalidArgumentException("Provider [{$name}] is not supported.") if truly unknown.

Test plan

• All 11 built-in drivers still return correct PrismProvider enum values
• Unknown drivers (e.g. 'bedrock', 'workers-ai') return the driver name as a string
• All 21 existing unit tests pass (91 assertions)

Closes #283