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

datasette/utils/__init__.py

@@ -1294,6 +1294,27 @@ async def derive_named_parameters(db: "Database", sql: str) -> List[str]:
     return named_parameters(sql)
 
 
+def error_body(messages, status):
+    """
+    The canonical JSON error body used by every Datasette JSON error response:
+
+        {"ok": False, "error": "...", "errors": ["...", ...], "status": 400}
+
+    "error" is all of the messages joined with "; ", "errors" is the full
+    list, "status" matches the HTTP status code. Callers may add extra
+    context keys to the returned dictionary but must not remove these four.
+    """
+    if isinstance(messages, str):
+        messages = [messages]
+    messages = [str(message) for message in messages]
+    return {
+        "ok": False,
+        "error": "; ".join(messages),
+        "errors": messages,
+        "status": status,
+    }
+
+
 def add_cors_headers(headers):
     headers["Access-Control-Allow-Origin"] = "*"
     headers["Access-Control-Allow-Headers"] = "Authorization, Content-Type"