Adopt larastan and resolve PHPStan level 2 errors

[pushpak1300]

Bumping phpstan to level 2 surfaced 180 errors, most of them Laravel-shaped: Storage::disk()->mimeType() on the Filesystem contract, higher-order collection proxies (->map->delta->join('')), facade returns, etc. Fixing those with vanilla phpstan means writing workarounds that obscure intent.

Switching to larastan resolves the Laravel-specific noise outright and surfaces only the real structural gaps in the codebase, which this PR addresses.

Real fixes (not larastan stubs)

A new base contract makes the four methods every provider exposes part of the type system instead of being only on the abstract Providers\Provider class:

// before: gateways accept FileProvider but call methods on the abstract class
$provider->name(); // PHPStan: method not found on FileProvider

// after: Contracts\Providers\Provider declares name(), driver(),
// providerCredentials(), additionalConfiguration(). Every capability
// contract extends it.

A new Contracts\RemembersConversation interface replaces class_uses_recursive sniffing for conversation-aware agents:

// before
if (in_array(RemembersConversations::class, class_uses_recursive($agent))
    && $agent->hasConversationParticipant()) { ... }

// after
if ($agent instanceof RemembersConversation
    && $agent->hasConversationParticipant()) { ... }

BC note: StoreProvider::addFileToStore gains an optional array $metadata = [] parameter to match the ManagesStores trait implementation. Third-party implementers will need to add it.

[danielhe4rt]

Thought all laravel packages we're enforced on phpstan lvl.5 minimum by default

this will break any contract?