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/providers/gemini.ts

@@ -31,6 +31,12 @@ import type {
 } from '../types/index.js';
 import { BaseAIProvider } from './base.js';
 import { AIProviderError, AIErrorType } from '../types/index.js';
+import {
+  DEFAULT_MAX_TOKENS,
+  DEFAULT_MODELS,
+  DEFAULT_TEMPERATURE,
+  VALIDATION_PROMPT
+} from '../constants.js';
 
 // ============================================================================
 // TYPES AND INTERFACES
@@ -95,7 +101,7 @@ export class GeminiProvider extends BaseAIProvider {
   constructor(config: GeminiConfig) {
     super(config);
 
-    this.defaultModel = config.defaultModel || 'gemini-2.5-flash';
+    this.defaultModel = config.defaultModel || DEFAULT_MODELS.gemini;
     this.safetySettings = config.safetySettings;
     this.defaultGenerationConfig = config.generationConfig;
 
@@ -207,7 +213,7 @@ export class GeminiProvider extends BaseAIProvider {
     try {
       await this.client.models.generateContent({
         model: this.defaultModel,
-        contents: [{ role: 'user', parts: [{ text: 'Hi' }] }],
+        contents: [{ role: 'user', parts: [{ text: VALIDATION_PROMPT }] }],
         config: { maxOutputTokens: 1 }
       });
     } catch (error: any) {
@@ -291,8 +297,8 @@ export class GeminiProvider extends BaseAIProvider {
     const defaults = this.defaultGenerationConfig;
 
     const config: GenerateContentConfig = {
-      temperature: params.temperature ?? defaults?.temperature ?? 0.7,
-      maxOutputTokens: params.maxTokens ?? defaults?.maxOutputTokens ?? 1000
+      temperature: params.temperature ?? defaults?.temperature ?? DEFAULT_TEMPERATURE,
+      maxOutputTokens: params.maxTokens ?? defaults?.maxOutputTokens ?? DEFAULT_MAX_TOKENS
     };
 
     const topP = params.topP ?? defaults?.topP;