encode: prioritize zstd and br over gzip in content negotiation (#7772)

* fix(encode): prioritize zstd and br over gzip in content negotiation

* test(encode): update unit tests to reflect new default priority ties

* fix(encode): move default preferences to dynamic encode handler and restore generic negotiation helper

* test(encode): call real Provision function in served-response test

* test(encode): rename served-response test to TestServeHTTPDefaultEncodingPreference

* refactor(encode): use slices.SortStableFunc and httptest.NewRecorder as recommended

* refactor(encode): simplify sorting with cmp.Compare and check request error in test

* test(encode): fix variable redeclaration in TestServeHTTPDefaultEncodingPreference

Fix 'no new variables on left side of :=' error by changing 'err :=' to 'err ='
on line 347, since err was already declared on line 332.

This fixes the build failure in the encode module tests.