fix(service-analytics): key the compare-filter test fake without a raw NUL byte

`check:nul-bytes` (run inside the ESLint job) rejected a literal 0x00 written
into the fake service's group key. A raw NUL makes grep/ripgrep treat the whole
file as binary and return zero matches, so the file drops out of code search
and every grep-based lint; git only scans the first 8000 bytes, so at offset
5440 it still diffed as text.

Keyed with `JSON.stringify(...)` joined by `|` instead, which needs no exotic
byte at all and is unambiguous for the multi-dimension case. Deliberately NOT
the production `mergeByDimensions` key, whose delimiter-free concatenation is
#4821 — the fake must not import a second defect into a test for this one.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_015Br2xsJsczFsTR9bvbh2Ny

Author: claude