fix(task): keep short numeric refs out of uuid fallback

[birdmanmandbir]

Summary

• Treat short numeric refs like 42 as short IDs only after a short-id miss.
• Preserve 8-char numeric fallback like 12345678 as the displayed UUID-prefix form.
• Add a regression test for 42 not resolving 42000000... when no short ID 42 exists.

Triage

Fixed

• Short numeric refs could fall through into UUID prefix matching after a short-id miss.
• Missing regression coverage for short numeric miss vs UUID-prefix collision.

Verification

• nix shell nixpkgs#cmake -c cmake --build build-test --target task_executable -j$(nproc)
• TASK_USE_PATH=1 PATH="$PWD/build-test/src:$PATH" python3 test/short_id.test.py
• TASK_USE_PATH=1 PATH="$PWD/build-test/src:$PATH" python3 test/filter_uuid_fastpath.test.py
• TASK_USE_PATH=1 PATH="$PWD/build-test/src:$PATH" python3 test/uuid.test.py
• git diff --check