fix: Missing LETZAI model

[daizhengxue]

Pull Request: Add LETZAI to ModelProviderName enum

What does this PR do?

Adds missing LETZAI enum value to ModelProviderName to fix type errors in agent/src/index.ts.

Changes needed:

In @elizaos/packages/core, add to ModelProviderName enum:

Testing

1. Build core package
2. Verify agent/src/index.ts compiles without errors
3. Verify LETZAI provider works with getTokenForProvider()

Risk Level: Low

• Simple enum addition
• Only affects type system
• No runtime behavior changes

Testing Steps

1. Add enum value
2. Run pnpm build in core package
3. Run pnpm build in agent package
4. Verify no TypeScript errors in agent/src/index.ts

No documentation changes needed as this is an internal enum update.