Unify JSON error responses into one canonical shape

All JSON error responses now use a single format built by the new
datasette.utils.error_body() helper:

    {"ok": false, "error": "...", "errors": ["..."], "status": 400}

- error is all messages joined with '; ', errors is the full list,
  status always matches the HTTP status code
- The exception handler no longer emits the legacy title key in JSON
  (it is still available to the HTML error template)
- The permission debug endpoints (/-/allowed, /-/rules, /-/check,
  POST /-/permissions) no longer return bare {"error": ...} objects
- JSON renderer SQL errors keep their rows/truncated context keys but
  now include the canonical keys as well
- _shape=object misuse (queries or tables without primary keys) now
  returns HTTP 400 instead of 200 with an error body
- Method-not-allowed 405 responses use the canonical shape

Adds tests/test_error_shape.py covering all four previous shape
producers, updates affected tests, and documents the format in a new
'Error responses' section of docs/json_api.rst.

Implements section 1 of stable-api-recommendations.md.

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01GrHZSypDfMnym1tM5XJAFZ

tests/test_api.py

@@ -323,20 +323,21 @@ def test_sql_time_limit(app_client_shorter_time_limit):
         "/fixtures/-/query.json?sql=select+sleep(0.5)",
     )
     assert 400 == response.status
+    expected_message = (
+        "<p>SQL query took too long. The time limit is controlled by the\n"
+        '<a href="https://docs.datasette.io/en/stable/settings.html#sql-time-limit-ms">sql_time_limit_ms</a>\n'
+        "configuration option.</p>\n"
+        '<textarea style="width: 90%">select sleep(0.5)</textarea>\n'
+        "<script>\n"
+        'let ta = document.querySelector("textarea");\n'
+        'ta.style.height = ta.scrollHeight + "px";\n'
+        "</script>"
+    )
     assert response.json == {
         "ok": False,
-        "error": (
-            "<p>SQL query took too long. The time limit is controlled by the\n"
-            '<a href="https://docs.datasette.io/en/stable/settings.html#sql-time-limit-ms">sql_time_limit_ms</a>\n'
-            "configuration option.</p>\n"
-            '<textarea style="width: 90%">select sleep(0.5)</textarea>\n'
-            "<script>\n"
-            'let ta = document.querySelector("textarea");\n'
-            'ta.style.height = ta.scrollHeight + "px";\n'
-            "</script>"
-        ),
+        "error": expected_message,
+        "errors": [expected_message],
         "status": 400,
-        "title": "SQL Interrupted",
     }
 
 
@@ -350,7 +351,7 @@ async def test_custom_sql_time_limit(ds_client):
         "/fixtures/-/query.json?sql=select+sleep(0.01)&_timelimit=5",
     )
     assert response.status_code == 400
-    assert response.json()["title"] == "SQL Interrupted"
+    assert response.json()["error"].startswith("<p>SQL query took too long.")
 
 
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