test: cover config loader env interpolation

[saurabhhhcodes]

Summary\n- add Jest coverage for default config fallback when no apidrift.config.json exists\n- verify `` placeholders interpolate from process.env\n- verify missing env vars resolve to an empty string while using temp working directories\n\n## Validation\n- npm test -- --runInBand\n- node --check tests/config/loader.test.js\n- git diff --check\n\nCloses #2

Suggested GSSoC labels

If accepted for scoring, this should fit gssoc, level:beginner, dx, type:testing, quality:clean based on the linked issue labels and the focused validation in this PR.

[saurabhhhcodes]

Quick status for review: this PR is mergeable from GitHub's side and intentionally scoped to the linked apidrift-cli issue.

Validation already included in the PR body:

• focused Jest coverage for the touched behavior
• syntax check for the touched test/source file where applicable
• git diff --check

I also added the suggested GSSoC scoring labels in the PR body so maintainers can apply them directly if the change is accepted.

[saurabhhhcodes]

Hi @tanmayjoddar, this PR is still open, mergeable, and clean against the current base branch. I rechecked the repo test baseline locally (node --experimental-vm-modules node_modules/jest/bin/jest.js -> 3 suites / 11 tests passed). Please review when you get a chance, and add the GSSoC labels on the PR to mirror the linked issue if you plan to merge it.

[saurabhhhcodes]

Fresh validation pass on the current branch:\n- node --experimental-vm-modules node_modules/jest/bin/jest.js --runInBand -> 3 suites / 9 tests passed\n- node --check tests/config/loader.test.js\n- git diff --check\n\nThis remains clean and mergeable from my side; no GitHub checks are configured on the branch, so local validation is the current proof.