ci(behavior): speed up PRs with chromium-only matrix and dedup math spec imports (#2940)

* ci(behavior): chromium-only on PRs, full matrix on merge_group

Firefox is the wall-clock bottleneck at ~13 min per shard; gating it
behind merge_group and workflow_dispatch gives PRs fast chromium
feedback while still running the full 3-browser suite before merge.

* test(behavior): collapse math-equations spec to one test per fixture

The file loaded the same 11 DOCX fixtures 74 times - once per test.
In a recent Firefox run that single file accounted for ~809s of
passed-test time. Each describe block now loads its fixture once
and wraps the original tests as test.step() assertions, taking the
import count from 74 to 11 while preserving every expect().

* ci(behavior): run full matrix on push to main/stable as post-merge safety net

Pairs with the chromium-only PR matrix: Firefox/WebKit still run on
every change, just after merge instead of before. Measured approach
before considering a merge queue gate.

* Revert "ci(behavior): run full matrix on push to main/stable as post-merge safety net"

This reverts commit 785a24a.

Author: caio-pizzol

.github/workflows/ci-behavior.yml

@@ -29,7 +29,9 @@ jobs:
     strategy:
       fail-fast: false
       matrix:
-        browser: [chromium, firefox, webkit]
+        # PRs run chromium only for fast feedback. merge_group and workflow_dispatch
+        # run the full 3-browser matrix before anything lands on main.
+        browser: ${{ fromJSON(github.event_name == 'pull_request' && '["chromium"]' || '["chromium", "firefox", "webkit"]') }}
         shard: [1, 2, 3, 4]
     steps:
       - uses: actions/checkout@v6