Enable runtime access to provider name

[wpfleger96]

This PR adds runtime access to provider names by introducing a simple name field and instance method to the Provider trait. This enables session management code to identify which provider is currently in use, laying the groundwork for provider/model configuration persistence across session resumption.

Session management code interacts with providers as trait objects (&dyn Provider). To persist provider/model configuration when saving a session, we need to identify which provider is currently in use. Previously, provider metadata was only accessible via static methods, which can't be called on trait objects. This blocked the ability to save and restore the correct provider during session resumption.

The implementation adds a simple name field to each provider that gets initialized from their static metadata. For standard providers (like OpenAI, Anthropic), the name comes directly from their metadata. For custom providers (like "custom_deepseek"), the name is their custom identifier—not their underlying engine's name. This ensures custom providers are correctly distinguished from their base implementations.

Implementation details:

• Added get_name(&self) -> &str instance method to Provider trait
• Updated all 21 provider implementations to store and return a name: String field
• Standard providers initialize with name: Self::metadata().name
• Custom providers receive their name via from_custom_config(model, config, provider_name)
• Declarative provider registration passes the custom provider's name to ensure proper identification
• Updated all 7 mock providers in tests to implement the new trait method

Relates to #5358