✅ Add test coverage for strict_content_type with charset parameters

Verify that Content-Type headers with charset parameters (e.g.,
application/json; charset=utf-8) are correctly accepted by the
strict_content_type parsing logic, including vendor JSON media types
like application/vnd.api+json; charset=utf-8.

Co-Authored-By: Claude Opus 4.6 <[email protected]>

tests/test_strict_content_type_charset.py

@@ -0,0 +1,55 @@
+from fastapi import FastAPI
+from fastapi.testclient import TestClient
+
+app = FastAPI()
+
+
+@app.post("/items/")
+async def create_item(data: dict):
+    return data
+
+
+client = TestClient(app)
+
+
+def test_json_content_type_with_charset_utf8():
+    """Content-Type: application/json; charset=utf-8 should be accepted."""
+    response = client.post(
+        "/items/",
+        content='{"key": "value"}',
+        headers={"Content-Type": "application/json; charset=utf-8"},
+    )
+    assert response.status_code == 200
+    assert response.json() == {"key": "value"}
+
+
+def test_vendor_json_content_type_with_charset_utf8():
+    """Content-Type: application/vnd.api+json; charset=utf-8 should be accepted."""
+    response = client.post(
+        "/items/",
+        content='{"key": "value"}',
+        headers={"Content-Type": "application/vnd.api+json; charset=utf-8"},
+    )
+    assert response.status_code == 200
+    assert response.json() == {"key": "value"}
+
+
+def test_json_content_type_with_charset_iso_8859_1():
+    """Content-Type: application/json; charset=iso-8859-1 should be accepted."""
+    response = client.post(
+        "/items/",
+        content='{"key": "value"}',
+        headers={"Content-Type": "application/json; charset=iso-8859-1"},
+    )
+    assert response.status_code == 200
+    assert response.json() == {"key": "value"}
+
+
+def test_text_plain_content_type_is_rejected():
+    """Content-Type: text/plain should be rejected with 422."""
+    response = client.post(
+        "/items/",
+        content='{"key": "value"}',
+        headers={"Content-Type": "text/plain"},
+    )
+    assert response.status_code == 422