refactor: extract constants, parameterize validators, simplify factory

Quick-win refactors from the Refactoring Guru catalog:

- Replace Magic Number with Symbolic Constant: extract DEFAULT_TIMEOUT_MS,
  DEFAULT_MAX_RETRIES, DEFAULT_MAX_TOKENS, DEFAULT_TEMPERATURE, default
  models per provider, and validation prompt into src/constants.ts.
- Parameterize Method: collapse validateTemperature / validateTopP /
  validateMaxTokens (and the inline timeout/maxRetries checks) into a
  single validateNumberInRange helper with bounds metadata.
- Inline Class / Remove Middle Man: drop the unused ProviderRegistry
  class from utils/factory.ts. createProvider now dispatches through
  the PROVIDER_REGISTRY const directly instead of a parallel switch.

Also fixes stale VERSION constant in src/index.ts (was 1.3.1).

src/index.ts

@@ -58,4 +58,4 @@ export const SUPPORTED_PROVIDERS = ['claude', 'openai', 'gemini', 'openwebui'] a
 /**
  * Package version
  */
-export const VERSION = '1.3.1'; 
+export const VERSION = '2.0.0';