* fix(api): exempt test-model requests from Output Styles injection (#6240)
Root cause: handleChatCore's Phase 4A Output Styles injection (chatCore.ts)
was gated only by the operator's global compression.enabled switch,
independent of the per-request x-omniroute-compression header. The
dashboard 'Test model' action (modelTestRunner.ts) never sent that header,
so a globally-enabled Output Style (e.g. 'Ultra terse') always leaked its
system-prompt injection into a plain connection test.
Fix: skip Output Styles injection when the request explicitly opts out via
x-omniroute-compression: off, and always send that header from
buildInternalChatRequest / buildInternalRerankRequest.
Regression guard: tests/integration/test-model-compression-off-6240.test.ts,
tests/unit/model-test-runner-compression-off-6240.test.ts
* chore: sync CHANGELOG to release tip (#6511; bullet re-added at merge)
