ci(proxy-mgmt-behavior): add PR-triggered workflow for the behavior suite

Slice 13 of the management-endpoints behavior-pinning effort. New
workflow ``test-unit-proxy-mgmt-behavior.yml`` fires ``on: pull_request``
for the same branch set every other proxy unit-test workflow watches
(main, litellm_internal_staging, litellm_oss_branch, litellm_**).

It delegates to the existing reusable ``_test-unit-services-base.yml``
with ``enable-postgres: true``, which already provisions a postgres:14
service container and runs ``prisma db push`` against it before pytest
collects. ``reruns: 0`` because a behavior-pinning matrix that needs
reruns is itself a regression — flakes are signal.

``timeout-minutes: 15`` gives generous headroom over the local 5.9s
whole-suite wall-time; the binding G2 budget is 10 min.

Plan: https://www.notion.so/36643b8acdab8128a581ced0f6a4744d

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

@@ -0,0 +1,29 @@
+name: "Unit Tests: Proxy Management-Endpoint Behavior Pinning"
+
+on:
+  pull_request:
+    branches:
+      - main
+      - litellm_internal_staging
+      - litellm_oss_branch
+      - "litellm_**"
+
+permissions:
+  contents: read
+  id-token: write
+  pull-requests: write
+
+concurrency:
+  group: ${{ github.workflow }}-${{ github.event.pull_request.number || github.ref }}
+  cancel-in-progress: true
+
+jobs:
+  proxy-mgmt-behavior:
+    uses: ./.github/workflows/_test-unit-services-base.yml
+    with:
+      test-path: tests/proxy_behavior
+      workers: 2
+      reruns: 0
+      enable-postgres: true
+      artifact-name: proxy-mgmt-behavior
+      timeout-minutes: 15