replace magic strings with constants for model provider IDs

[NeverMorewd]

• Add ModelProviderIds static class with constants for all provider IDs
• Update SupportedTemplates to use constants instead of string literals

[DearVa]

SupportedTemplates is more like a hard-coded configuration file where IDs are not reused, so encapsulating it offers no benefits.

[DearVa]

Maybe a enum is better? But I don't know if it may broke the settings save/load.

[NeverMorewd]

@DearVa

We could technically preserve compatibility by keeping the Id as a string and adding an IdEnum field (ignored during serialization) for internal logic checks. However, after looking into the code, this approach seems unnecessarily complex and may introduce future risks — especially if we later support user-defined provider IDs. In that case, maintaining a conversion between enums and custom strings would become a hack-prone workaround.