fix: allow configurable request timeout

[saurabhhhcodes]

Summary

• closes dx: allow configuring HTTP timeout via env var #7
• supports APIDRIFT_TIMEOUT_MS for slower APIs while keeping the default at 10000 ms
• ignores invalid, non-integer, or non-positive timeout values
• documents the environment variable in the README
• adds focused unit tests for timeout parsing behavior

Validation

• npm ci
• npm test -- --runInBand (13 passed)
• node --check src/core/fetcher.js
• node --check tests/core/fetcher.test.js
• git diff --check

Suggested labels: gssoc, dx, good first issue.

Suggested GSSoC labels

If accepted for scoring, this should fit gssoc, level:beginner, dx, type:feature, 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 / 13 tests passed\n- node --check src/core/fetcher.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.