test(editor-v2): pin form-POST round-trip for blueprint config

New test_blueprint_config_form_post_round_trip — POSTs a multi-line
config, GETs the page, asserts each line re-renders inside the
textarea. Pins the round-trip the v2 editor's submit-time copy
handler must preserve before any template wiring lands.

Skipped a corresponding test_overlay_script_form_post_contract test
— the existing test_admin_creates_system_wide_script_overlay at
test_script_overlay_routes.py:~270 already asserts
overlay.script == "echo admin" after a POST /overlays/<id>/script,
which is the same form-contract pin. YAGNI; no need to duplicate.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

l4d2web/tests/test_blueprints.py

@@ -411,3 +411,45 @@ def test_blueprint_detail_picker_emits_hidden_overlay_ids_in_order(user_client)
     second = body.find('name="overlay_ids" value="1"')
     assert first != -1 and second != -1
     assert first < second
+
+
+def test_blueprint_config_form_post_round_trip(user_client) -> None:
+    """Pin the form-POST contract for the `config` textarea: a multi-line
+    value sent in the POST body re-renders inside the textarea on the
+    subsequent GET. The CodeMirror 6 editor wires a submit-time copy
+    handler that writes `controller.getValue()` into `textarea.value`
+    before the browser serializes the form; if that handler regresses,
+    this test breaks before the e2e suite even runs."""
+    create = user_client.post(
+        "/blueprints",
+        data={
+            "name": "form-contract",
+            "arguments": "",
+            "config": "",
+            "overlay_ids": [],
+        },
+        headers={"X-CSRF-Token": "test-token"},
+    )
+    assert create.status_code == 302
+    blueprint_path = create.headers["Location"]
+
+    pinned_config = "// pinned by form-contract test\nsv_cheats 0\nexec server.cfg\n"
+    update = user_client.post(
+        blueprint_path,
+        data={
+            "name": "form-contract",
+            "arguments": "",
+            "config": pinned_config,
+            "overlay_ids": [],
+        },
+        headers={"X-CSRF-Token": "test-token"},
+    )
+    assert update.status_code == 302
+
+    page = user_client.get(blueprint_path)
+    assert page.status_code == 200
+    body = page.get_data(as_text=True)
+    # The route may normalize trailing whitespace; assert each non-empty
+    # line round-tripped into the rendered textarea.
+    for line in ("// pinned by form-contract test", "sv_cheats 0", "exec server.cfg"):
+        assert line in body, f"line not found in rendered page: {line!r}"