fix(client): remove caller abort listener after successful fetch

[MukundaKatta]

Summary

• remove the forwarded abort listener from the caller signal once fetchWithTimeout completes
• avoid leaving an orphaned listener attached to successful requests
• add regression coverage for listener cleanup on the success path

Testing

• npm test -- --runInBand tests/index.test.ts --testNamePattern="custom signal|removes the caller abort listener"
• npx prettier --check src/client.ts tests/index.test.ts