refactor: split parsers.test.ts into one test file per package manager

[sonukapoor]

tests/parsers.test.ts currently contains all parser tests for every package manager (npm, pnpm, yarn classic, yarn berry, bun, npm lock graph). As coverage grows this file becomes hard to navigate and violates the principle of one test file per unit.

Split into dedicated test files:

• tests/parsers/npm-lock.test.ts
• tests/parsers/npm-lock-graph.test.ts
• tests/parsers/pnpm-lock.test.ts
• tests/parsers/yarn-lock.test.ts
• tests/parsers/bun-lock.test.ts
• tests/parsers/package-json.test.ts
• tests/parsers/load-packages.test.ts

No behavior changes — pure test file reorganization.