feat: Upwards-recursively read .env.braintrust containing BRAINTRUST_API_KEY

[Luca Forstner (lforst)]

Direct port of braintrustdata/braintrust-sdk-javascript#2049

Prompt used + some cleanup:

Implement `.env.braintrust` API key discovery for this Braintrust SDK.

Context:
The Braintrust instrumentation wizard will generate a file named `.env.braintrust` in the user’s current working directory. That file contains a dotenv-style `BRAINTRUST_API_KEY=...` entry. The goal is that after running the wizard, users can immediately run or verify local Braintrust instrumentation without manually exporting `BRAINTRUST_API_KEY`.

`.env.braintrust` is not a general dotenv loader. It is only a credential fallback for Braintrust SDK API-key lookup.

Required behavior:
- Preserve precedence:
  1. Explicit caller-provided API key wins.
  2. Nonblank `BRAINTRUST_API_KEY` from the process environment wins.
  3. Otherwise, read `BRAINTRUST_API_KEY` from `.env.braintrust`.
- Treat missing, empty, or whitespace-only environment values as unset.
- Look for `.env.braintrust` starting at the current working directory at lookup time, then walk upward.
- Cap lookup at cwd plus 64 parent directories.
- The nearest `.env.braintrust` wins.
- The nearest `.env.braintrust` is a boundary: if it exists but has no nonblank `BRAINTRUST_API_KEY`, do not check higher parents.
- If the nearest `.env.braintrust` cannot be read, return “not found” and do not check higher parents. Do not throw from credential discovery.
- Read only `BRAINTRUST_API_KEY`; do not load or expose other variables from the file.
- Do not mutate the process environment.
- Support normal dotenv syntax if the SDK/runtime has a standard parser: quotes, comments, and `export BRAINTRUST_API_KEY=...`.

Implementation guidance:
- Prefer lazy, nonblocking lookup.
- If possible, start candidate file reads in parallel, but preserve nearest-wins semantics: a higher parent may only win after all closer candidates are known absent.
- If a constructor/setup path is synchronous, do not add blocking file IO there.
- For telemetry/exporter integrations, make export/flush wait for API-key discovery when needed.

Look at the reference implementation PR: https://github.com/braintrustdata/braintrust-sdk-javascript/pull/2049

[Abhijeet Prasad (AbhiPrasad)]

gonna take this PR over w/ some of the env variable refactors I made.