fix(PocketIC): simplify PocketIc::try_get_controllers #27360

Job	Run time
Bazel Build Fuzzers	1m 21s
Bazel Test All	8m 9s
Bazel Build Fuzzers AFL	2m 15s
Bazel Test macOS Intel	0s
Build IC	7m 46s
Cargo Build Release Linux	9m 11s
Cargo Clippy Linux	4m 36s
Python CI Tests	1m 42s
Build Determinism (//publish/binaries:upload, release, build-ic/release, false)	9s
Build Determinism (//publish/canisters:upload, canisters, build-ic/canisters, false)	9s
Build Determinism (//ic-os/guestos/envs/prod:upload_disk-img, guest-os/update-img, build-ic/icos/...	12s
Build Determinism (//ic-os/hostos/envs/prod:upload_update-img, host-os/update-img, build-ic/icos/...	9s
Build Determinism (//ic-os/setupos/envs/prod:upload_disk-img, setup-os/disk-img, build-ic/icos/se...	8s
	35m 47s

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