style: format portal.ts and portal.test.ts with Biome

Apply the repo's new Biome config (main: 29f9da1) to the files this PR
touches: `biome check --write` plus manual fixes for the three
noAssignInExpressions errors (behavior-preserving rewrites). Both files
now pass `biome check` with no errors; all 56 portal tests pass.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

Author: mo4islona