Answer workspace refusals instead of crashing on them

json_error takes (status_code, message). Every refusal in the two
workspace routers passed them the other way round, which builds a
JSONResponse with a string status and raises TypeError inside
Response.init_headers before a byte is written. The branch meant to
explain a limit to the user returned 500 instead.

A member already holding the default two workspaces therefore got a 500
from the workspace page's Open workspace button rather than the quota
message. Twelve call sites corrected across both routers.

workspace_open also let ContainerError escape as a second 500 on the
same button when the ppy image is not built; it now returns 400 through
the shared fail helper.

tests/unit/responses.py AST-scans the whole package for both argument
orders, so the swap cannot reappear anywhere.

tests/unit/responses.py

@@ -0,0 +1,50 @@
+# retoor <retoor@molodetz.nl>
+
+import ast
+from pathlib import Path
+
+PACKAGE = Path(__file__).resolve().parents[2] / "devplacepy"
+
+
+def _json_error_calls():
+    for path in sorted(PACKAGE.rglob("*.py")):
+        tree = ast.parse(path.read_text(), filename=str(path))
+        for node in ast.walk(tree):
+            if not isinstance(node, ast.Call):
+                continue
+            name = node.func.attr if isinstance(node.func, ast.Attribute) else getattr(node.func, "id", "")
+            if name == "json_error" and node.args:
+                yield path.relative_to(PACKAGE.parent), node
+
+
+def test_json_error_never_receives_a_message_as_its_status():
+    offenders = [
+        f"{path}:{node.lineno}"
+        for path, node in _json_error_calls()
+        if isinstance(node.args[0], ast.Constant) and isinstance(node.args[0].value, str)
+    ]
+    assert not offenders, (
+        "json_error(status_code, message) - a string in the status position builds a "
+        f"JSONResponse with a non-int status and raises TypeError at request time: {offenders}"
+    )
+
+
+def test_json_error_never_receives_a_status_as_its_message():
+    offenders = [
+        f"{path}:{node.lineno}"
+        for path, node in _json_error_calls()
+        if len(node.args) > 1
+        and isinstance(node.args[1], ast.Constant)
+        and isinstance(node.args[1].value, int)
+    ]
+    assert not offenders, (
+        "json_error(status_code, message) - an int in the message position means the "
+        f"arguments were swapped: {offenders}"
+    )
+
+
+def test_every_json_error_status_is_a_real_http_error_code():
+    for path, node in _json_error_calls():
+        first = node.args[0]
+        if isinstance(first, ast.Constant) and isinstance(first.value, int):
+            assert 400 <= first.value <= 599, f"{path}:{node.lineno} status {first.value}"