docs(proxy_behavior): suite README with local-repro + conventions + gates

Slice 16 of the management-endpoints behavior-pinning effort. The README
documents:

  * The same three commands the CI workflow runs locally (BYO-DATABASE_URL,
    no new tooling).
  * Suite layout — what each test file covers, which slice it lands.
  * The asyncio loop_scope convention required for session fixtures
    (httpx AsyncClient + prisma connection) to share a loop with each
    test body.
  * G3 strict-import convention + the test that enforces it.
  * Read-world vs scratch-world fixture conventions.
  * Behavior-pinning philosophy: pin observed codes; flag, don't judge.
  * Where each G1–G5 + PR1.M1–M3 gate's evidence lives.

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

tests/proxy_behavior/management/README.md

@@ -0,0 +1,102 @@
+# Management-endpoint behavior-pinning suite
+
+HTTP-boundary regression tests for
+`litellm/proxy/management_endpoints/key_management_endpoints.py` (PR1 — Key
+Tier-1). Runs against the real proxy app via in-process `httpx.ASGITransport`,
+connected to a real Postgres pointed at by `DATABASE_URL`. **No mocks** —
+auth runs, prisma runs, integrations run. Test bodies make HTTP calls and
+assert at the API boundary.
+
+The eventual goal (across PR1–PR3) is to pin every authorization /
+cross-tenant / budget-bypass boundary on the key + team management
+surfaces. PR1 covers six Tier-1 key endpoints (`/key/generate`, `/key/info`,
+`/key/list`, `/key/update`, `/key/regenerate`, `/key/delete`). See the
+[Notion plan](https://www.notion.so/36643b8acdab8128a581ced0f6a4744d) for
+the full scope.
+
+## Local repro
+
+Identical to the three commands the CI workflow
+(`.github/workflows/test-unit-proxy-mgmt-behavior.yml`, which delegates
+to `_test-unit-services-base.yml`) runs:
+
+```bash
+# 1. Bring up Postgres
+docker run --rm -d --name litellm-test-pg \
+  -e POSTGRES_USER=litellm -e POSTGRES_PASSWORD=litellm -e POSTGRES_DB=litellm_test \
+  -p 5432:5432 postgres:14
+
+# 2. Migrate the schema (one-time per fresh DB)
+export DATABASE_URL=postgresql://litellm:litellm@localhost:5432/litellm_test
+uv run prisma generate --schema litellm/proxy/schema.prisma
+uv run prisma db push --schema litellm/proxy/schema.prisma --accept-data-loss
+
+# 3. Run the suite
+uv run pytest tests/proxy_behavior/management/
+```
+
+Whole-suite wall-time is ~6s on a warm cache (one ~1.5s session setup +
+~0.01–0.04s per test). Re-running back-to-back produces identical pass
+counts — the scratch-namespace teardown leaves no rows behind.
+
+### Single scenario / inner loop
+
+```bash
+uv run pytest tests/proxy_behavior/management/test_key_update.py -k self/owner -v
+```
+
+## Layout
+
+```
+tests/proxy_behavior/management/
+├── conftest.py                    # session ASGI client, world seed, scratch fixture
+├── actors.py                      # 8-actor enum + seed_world() helper
+├── test_smoke.py                  # liveness + key/generate de-risk smoke
+├── test_world_seed.py             # every seeded actor key authenticates
+├── test_scratch_teardown.py       # scratch namespace cleanup invariants
+├── test_no_management_imports.py  # G3 — strict-import grep as a test
+├── test_key_generate.py           # Slice 7 — actor × target matrix
+├── test_key_info.py               # Slice 8
+├── test_key_list.py               # Slice 9
+├── test_key_update.py             # Slice 10
+├── test_key_regenerate.py         # Slice 11
+├── test_key_delete.py             # Slice 12
+├── regression_replay/README.md    # G4 — fix-PR → catching-scenario mapping
+└── mutmut_triage/pr1.md           # G5 — survivor classification protocol
+```
+
+## Conventions
+
+- **Async fixture / loop scope.** `pyproject.toml` sets
+  `asyncio_default_fixture_loop_scope = "session"`, but the default
+  *test* loop scope is per-function. Add
+  `pytestmark = pytest.mark.asyncio(loop_scope="session")` at the top
+  of every test file so the AsyncClient and prisma connection (both
+  session-scoped) share a loop with the test body.
+- **Forbidden imports (G3).** No `from litellm.proxy.management_endpoints`,
+  no `mock`/`patch` on `user_api_key_auth`. Enforced by
+  `test_no_management_imports.py` as a pytest item.
+- **Read-world vs scratch-world.** The `world` fixture seeds an immutable
+  read-world under the `behavior-pin-` prefix; tests must not mutate
+  those rows. The `scratch` fixture gives a per-test
+  `scratch-<uuid>` prefix and tears down any row tagged with it.
+  Write scenarios always tag their creates with `scratch.prefix`.
+- **Behavior pinning, not behavior judging.** Expected status codes are
+  pinned against current handler behavior. The suite's job is to make
+  *changes* to that behavior visible — not to assert what the codes
+  *should* be. Comments above each `_SCENARIOS` block call out
+  surprising or potentially-buggy behaviors for human review.
+
+## Gate evidence
+
+PR1's evidence for each G1–G5 + PR1.M1–M3 gate lives in:
+
+- **G1** — CI run on the PR's workflow `test-unit-proxy-mgmt-behavior` (green).
+- **G2** — `pytest --durations=…` summary in the PR description (≤ 10 min).
+- **G3** — `test_no_management_imports.py` is part of the suite itself.
+- **G4** — `regression_replay/README.md`.
+- **G5** — `mutmut_triage/pr1.md`, filled in after the first manually-triggered
+  `mutation-test.yml` run.
+- **PR1.M1** — total scenario count, this README's "Layout" section.
+- **PR1.M2** — this README + the workflow YAML are the local-repro contract.
+- **PR1.M3** — `mutmut_triage/pr1.md` "Baseline metrics" table.