Write endpoints parse the body as JSON regardless of Content-Type

The insert, upsert, alter and set-column-type endpoints previously
required Content-Type: application/json while /-/create parsed the body
blind - and insert returned a 500 AttributeError when the header was
missing entirely. The lenient rule is now uniform: the body is always
parsed as JSON and invalid JSON is a 400. This makes curl -d and
requests data=json.dumps(...) invocations work without remembering the
header. Cross-site request forgery remains prevented by the Origin and
Sec-Fetch-Site checks in CrossOriginProtectionMiddleware, which is the
defense the strict content-type requirement was historically standing
in for.

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

tests/test_api_write.py

@@ -394,13 +394,6 @@ async def test_insert_rows_post_body_too_large(tmp_path_factory):
                 "Invalid JSON: Expecting property name enclosed in double quotes: line 1 column 2 (char 1)"
             ],
         ),
-        (
-            "/data/docs/-/insert",
-            {},
-            "invalid_content_type",
-            400,
-            ["Invalid content-type, must be application/json"],
-        ),
         (
             "/data/docs/-/insert",
             [],
@@ -582,11 +575,7 @@ async def test_insert_or_upsert_row_errors(
         json=input,
         headers={
             "Authorization": "Bearer {}".format(token),
-            "Content-Type": (
-                "text/plain"
-                if special_case == "invalid_content_type"
-                else "application/json"
-            ),
+            "Content-Type": "application/json",
         },
     )