feat: configure HTTP timeout via env

[saurabhhhcodes]

Summary

• add APIDRIFT_HTTP_TIMEOUT_MS parsing with a safe 10s default and validation for positive integer values
• apply the shared timeout helper to endpoint fetches, OpenAPI discovery, and GraphQL discovery
• document the env var in .env.example and README
• add Jest coverage for default, configured, and invalid timeout values

Fixes #7

Validation

• git diff --check
• node --check src/utils/httpTimeout.js
• node --check src/core/fetcher.js
• node --check src/discovery/openapi.js
• node --check src/discovery/graphql.js
• npm test -- --runInBand could not run locally because npm is not available in this shell

GSSoC labels requested

Please add/keep gssoc and any required difficulty/type scoring labels if this is accepted.

[saurabhhhcodes]

Closing this older version to keep the review queue clean. The current replacement for the configurable timeout work is #25.