refactor: replace overloads with generics for streamable methods

[majiayu000]

Summary

• Replace function overloads with generic type parameters for streamable methods
• Allows stream parameter to accept boolean type while preserving correct return type inference
• Methods updated: generate, chat, create, pull, push

Problem

When using overloads, passing a boolean variable as the stream parameter caused TypeScript errors due to ambiguity in overload resolution:

// This caused type errors before
const generate = (request: GenerateRequest, stream: boolean) => {
  return ollama.generate({ ...request, stream })
}

Solution

Replace overloads with a generic type parameter that defaults to false:

async generate<S extends boolean = false>(
  request: GenerateRequest & { stream?: S },
): Promise<S extends true ? AbortableAsyncIterator<GenerateResponse> : GenerateResponse>

Test plan

• TypeScript compilation succeeds
• Existing tests pass
• New tests verify generic typing works correctly

Fixes #78

[BruceMacD]

This is looking good, thanks for the contribution. I'll test a bit more once the conflict is resolved.

We should note that this will effect any clients that extend ollama-js for some reason, although I am not aware of anyone using the library like that.