feat: add generic AI provider support (#64)

[Akash5908]

Description

Resolves #64

This PR introduces generic AI provider support to Passmark, allowing users to configure any Vercel AI SDK-compatible provider (e.g., corporate self-hosted proxies, LiteLLM, vLLM, Ollama, etc.) instead of being limited to the hardcoded gateway options.

It adopts a programmatic approach, allowing users to pass a factory function (createProvider) in their configure() setup.

Key Changes

• New CustomProviderConfig Type: Users can now register custom providers and provide optional model name aliases.
• Widened AIGateway: The gateway field now accepts arbitrary strings without breaking autocomplete for the default literals. This allows routing all models through a single custom gateway.
• Custom Provider Resolution (resolveModel):
  • resolveModel() now handles provider-prefix mode (e.g., llm-proxy/gpt-4) and gateway mode (all models routed through a specific custom provider).
  • Provider instances are lazily constructed and cached in memory to ensure createProvider is only executed once per provider.
• Threaded Configurations: Passmark correctly merges the providers map down from global configurations to per-call and per-step AIOverride instances.
• Documentation Updates: Added a "Custom Providers" section to README.md and custom proxy examples to .env.example.

Usage Example

import { configure } from "passmark";
import { createOpenAICompatible } from "@ai-sdk/openai-compatible";

configure({
  ai: {
    providers: {
      "llm-proxy": {
        createProvider: () => createOpenAICompatible({
          name: "llm-proxy",
          apiKey: process.env.LLM_PROXY_API_KEY!,
          baseURL: `${process.env.LLM_PROXY_API_BASE_URL}/api/v1/proxy/openai/auto`,
        }),
      },
    },
    models: {
      stepExecution: "llm-proxy/google/gemini-3.5-flash",
      assertionPrimary: "llm-proxy/anthropic/claude-haiku-4.5"
    },
  },
});

Testing Done

• Verified pnpm run build succeeds with no type errors.
• Verified all existing tests pass (pnpm test).
• Added src/__tests__/models.test.ts to test custom provider resolution, model aliasing, and caching logic.
• Added tests in src/__tests__/config.test.ts to verify overrides merging correctly.