ci(proxy-mgmt-behavior): drop xdist (workers=0) to fix seed race

First run on PR #28321 failed with UniqueViolation on
``behavior-pin-budget`` plus cascading missing-membership FK errors. Both
xdist workers entered ``seed_world()`` concurrently against the shared
Postgres service container; whichever lost the race left the world in a
half-seeded state and downstream tests ran against missing
team_membership rows.

Whole-suite wall-time is ~7s sequentially, so disabling xdist here costs
nothing — and the seed itself is the wrong place to add per-worker
isolation (the world is intentionally shared so set-equality assertions
in /key/list have a deterministic expected set).

.github/workflows/test-unit-proxy-mgmt-behavior.yml

@@ -22,7 +22,12 @@ jobs:
     uses: ./.github/workflows/_test-unit-services-base.yml
     with:
       test-path: tests/proxy_behavior
-      workers: 2
+      # workers=0 (no xdist): the world seed is a single shared Postgres
+      # state — two xdist workers both call seed_world() and race on the
+      # ``behavior-pin-budget`` row, producing UniqueViolation + cascading
+      # missing-membership FK failures. The whole suite is ~7s sequentially,
+      # so the cost of disabling parallelism here is negligible.
+      workers: 0
       reruns: 0
       enable-postgres: true
       artifact-name: proxy-mgmt-behavior