fix(buzz-agent): honor OPENAI_BASE_URL as fallback for base URL

[Chukwuebuka-2003]

Summary

When BUZZ_AGENT_PROVIDER is set to an OpenAI-compatible provider but OPENAI_COMPAT_BASE_URL is not explicitly set, the agent should fall back to the value of OPENAI_BASE_URL before using the default https://api.openai.com/v1. This matches user expectations when migrating from other tools that use OPENAI_BASE_URL as the standard env var.

Fixes #3630

Changes

• In config.rs, the from_env() OpenAI base URL resolution now checks OPENAI_BASE_URL as a fallback when OPENAI_COMPAT_BASE_URL is unset, before falling through to https://api.openai.com/v1.

Testing

Existing tests pass — this is a pure backward-compatible fallback addition.

[Bartok9]

Independent review (Bartok9) — 2026-07-30 vs current main

Thanks for pushing on #3630 — agreeing env fallback is needed.

Blocking nit: precedence is inverted vs the PR description

Description says: when OPENAI_COMPAT_BASE_URL is unset, fall back to OPENAI_BASE_URL.

Current patch:

env("OPENAI_BASE_URL")
    .or_else(|| env("OPENAI_COMPAT_BASE_URL")
    .unwrap_or_else(|| "https://api.openai.com/v1".into()),

That prefers OPENAI_BASE_URL over the Buzz-native OPENAI_COMPAT_BASE_URL, so a leftover generic OPENAI_BASE_URL in the shell can override an intentional OPENAI_COMPAT_BASE_URL (opposite of typical specific-over-generic ordering).

Suggested:

env("OPENAI_COMPAT_BASE_URL")
    .or_else(|| env("OPENAI_BASE_URL"))
    .unwrap_or_else(|| "https://api.openai.com/v1".into()),

Testing gap

No unit test pins the two-env matrix (compat set / base only / neither). Worth a small from_env test with tempered env.

Scope note

Issue title mentions Agent defaults (Desktop). Env fallback still helps if defaults export OPENAI_BASE_URL into the agent process; if Desktop only writes another key, a second seam may remain — happy to help chase that if this lands and dogfood still breaks.

I can open a tiny residual PR with corrected precedence + tests if you want — won’t stomp if you push a fix here first.

[Bartok9]

Follow-up residual with corrected precedence + unit tests: #3773 — happy to close it if you flip the or_else order here and add similar tests on this branch first.