fix: remove abort listener after fetch settles

[MukundaKatta]

Fixes #1811.

Summary

• remove the forwarded caller abort listener after fetchWithTimeout settles
• keep the existing internal timeout cleanup behavior
• add a regression test that verifies the caller signal listener is detached after a successful request

Verification

• yarn test tests/index.test.ts --runInBand
• npx -p node@20 -c './node_modules/.bin/prettier --check src/client.ts tests/index.test.ts && ./node_modules/.bin/eslint src/client.ts tests/index.test.ts'
• npx -p node@20 -c './scripts/build'

Note: full yarn lint proceeds through Prettier, ESLint, and build under Node 20, then local type-checking fails because optional example dependencies are not installed in this checkout: @azure/identity, express, and next.