fix(ci): clear leftover .venv before Python 3.14 venv create (BUG-677)

Staging gate run 2594 died in Install dependencies with FileExistsError on a reused hostexecutor .venv. Remove the leftover directory and create the venv with --clear so Python 3.14 mkdir does not abort the job.

frontend/tests/staging-backend-workflows.test.ts

@@ -227,7 +227,11 @@ test("Gitea quality gate validates before publishing an immutable ACR manifest",
   assert.equal((workflow.match(/exact staging gate checkout failed after \$attempt bounded attempts/g) ?? []).length, 2);
   assert.equal((workflow.match(/\[\[ "\$fetch_succeeded" == true \]\]/g) ?? []).length, 2);
   assert.doesNotMatch(workflow, /http\.lowSpeedLimit=1024|http\.lowSpeedTime=30/);
-  assert.match(workflow, /python3 -m venv \.venv/);
+  // 原值: python3 -m venv .venv
+  // 新值: rm -rf leftover then python3 -m venv --clear .venv
+  // 原因: BUG-677 Python 3.14 FileExistsError on reused hostexecutor .venv
+  assert.match(workflow, /rm -rf -- \.venv\n\s+python3 -m venv --clear \.venv/);
+  assert.doesNotMatch(workflow, /python3 -m venv \.venv(?:\s|$)/);
   assert.match(workflow, /PIP_INDEX_URL: https:\/\/mirrors\.aliyun\.com\/pypi\/simple\//);
   assert.match(workflow, /NPM_CONFIG_REGISTRY: https:\/\/registry\.npmmirror\.com/);
   for (const constraint of [
@@ -272,6 +276,8 @@ test("Gitea quality gate validates before publishing an immutable ACR manifest",
   const installStep = workflow.match(
     /- name: Install dependencies[\s\S]*?(?=\n\s+- name: Validate backend, package, frontend, and database contracts)/,
   )?.[0] ?? "";
+  assert.match(installStep, /rm -rf -- \.venv/);
+  assert.match(installStep, /python3 -m venv --clear \.venv/);
   assert.match(installStep, /workdir="\$\(pwd -P\)"/);
   assert.match(installStep, /docker run --rm/);
   assert.doesNotMatch(installStep, /timeout --signal=TERM --kill-after=30s 900s docker run/);