Async prompt loading support

[AbhiPrasad]

Summary

Add native async support for prompt loading in the Python SDK via an `aload_prompt()` function, allowing users to load multiple prompts in parallel using `asyncio.gather` without blocking.

Context

• Use case: Users running async applications (e.g., with `asyncio.gather`) find that `load_prompt()` blocks, causing sequential execution when loading multiple prompts. This adds latency in production scenarios where prompts need to be fetched at request time.

• Current workaround: Use `run_in_executor` to wrap the sync call:

  loop = asyncio.get_event_loop()
  prompt = await loop.run_in_executor(
      None,
      lambda: braintrust.load_prompt(project=project, slug=slug)
  )

• Component: Python SDK

Prior Work

A prototype was built on branch async-load-prompt with usage:

await braintrust.aload_prompt(project="...", slug="...")

This was not merged at the time, but multiple customers have since requested async support.

Requirements

Must have:

• aload_prompt() function with same parameters as load_prompt()
• Works with asyncio.gather for parallel prompt loading
• Consistent behavior with sync version (caching, fallback, etc.)

Related

• Linear issue: https://linear.app/braintrustdata/issue/BRA-3922/python-sdk-async-prompt-loading-support
• Related PR: Async load prompt braintrust-sdk-javascript#771