test: add unit tests for config loader env interpolation

[tanmayjoddar]

Why

src/config/loader.js interpolates ${VAR} from environment variables. This is core to auth/header UX, but currently untested.

Where

• src/config/loader.js
• Add: tests/config/loader.test.js (or similar)

Acceptance Criteria

• When apidrift.config.json does not exist, loadConfig() returns defaults (empty environments + endpoints + heuristic discovery)
• When config contains ${STAGING_TOKEN}, it is replaced by process.env.STAGING_TOKEN
• When env var is missing, interpolation produces an empty string (current behavior)
• Tests set up and clean up temp working directory (no real files in repo root)
• npm test passes

[saurabhhhcodes]

Hi @tanmayjoddar, I would like to work on this under GSSoC'26.\n\nI will keep it scoped to unit tests for src/config/loader.js: default config when no file exists, `` interpolation from env, missing env vars resolving to an empty string, and temp-directory cleanup so repo files are untouched. I will run the full Jest suite before opening the PR.

[manya-singh7]

@tanmayjoddar I'd like to be assigned to this. I am a GSSoC 2026 contributor.