refactor: split OpenWebUI into strategy classes by backend

Replace Conditional with Polymorphism from the Refactoring Guru catalog.

The OpenWebUIProvider previously contained two parallel implementations
(completeWithChat / completeWithOllama, streamWithChat / streamWithOllama,
plus branching in validateConnection) selected by a `useOllamaProxy`
boolean. The 987-line file is split into:

  - openwebui-types.ts       wire-format response types
  - openwebui-http.ts        shared HTTP client (auth, timeout)
  - openwebui-strategies.ts  OpenWebUIStrategy interface plus
                             OpenWebUIChatStrategy and
                             OpenWebUIOllamaStrategy implementations
  - openwebui.ts             thin provider that picks one strategy at
                             construction and delegates

OpenWebUIProvider and OpenWebUIConfig (including useOllamaProxy) stay
in their original locations, so consumer imports are unchanged.

Side effects:
- Error mapping now goes through the base mapProviderError / providerErrorMessages
  hooks added in R2, removing handleOpenWebUIError and its duplicated status switch.
- providerInfo.version bumped to 2.0.0 to reflect the rewrite.

src/providers/openwebui-http.ts

@@ -0,0 +1,59 @@
+import { AIProviderError, AIErrorType } from '../types/index.js';
+import { DEFAULT_TIMEOUT_MS } from '../constants.js';
+
+export interface OpenWebUIHttpOptions {
+  baseUrl: string;
+  apiKey?: string;
+  timeout?: number;
+  dangerouslyAllowInsecureConnections?: boolean;
+}
+
+/**
+ * Thin HTTP client shared by OpenWebUI strategies.
+ * Handles auth header, request body serialization, and timeout-to-AIProviderError translation.
+ */
+export class OpenWebUIHttpClient {
+  readonly baseUrl: string;
+  private readonly apiKey: string | undefined;
+  private readonly timeout: number;
+  private readonly dangerouslyAllowInsecureConnections: boolean;
+
+  constructor(options: OpenWebUIHttpOptions) {
+    this.baseUrl = options.baseUrl;
+    this.apiKey = options.apiKey;
+    this.timeout = options.timeout ?? DEFAULT_TIMEOUT_MS;
+    this.dangerouslyAllowInsecureConnections = options.dangerouslyAllowInsecureConnections ?? true;
+  }
+
+  async request(path: string, method: string, body?: unknown): Promise<Response> {
+    const headers: Record<string, string> = {
+      'Content-Type': 'application/json',
+      'User-Agent': 'simple-ai-provider/2.0.0'
+    };
+
+    if (this.apiKey) {
+      headers['Authorization'] = `Bearer ${this.apiKey}`;
+    }
+
+    const requestOptions: RequestInit = {
+      method,
+      headers,
+      body: body !== undefined ? JSON.stringify(body) : undefined,
+      signal: AbortSignal.timeout(this.timeout)
+    };
+
+    try {
+      return await fetch(`${this.baseUrl}${path}`, requestOptions);
+    } catch (error: any) {
+      if (error.name === 'AbortError') {
+        throw new AIProviderError(
+          'Request timed out',
+          AIErrorType.TIMEOUT,
+          undefined,
+          error
+        );
+      }
+      throw error;
+    }
+  }
+}