[BUG] Unbounded execution for write threads when parsing a flat_object field provided as array

[mgodwan]

Describe the bug

Unbounded execution for write threads when parsing a flat_object field provided as array.
Ideally, this should error out gracefully.

Related component

Indexing

To Reproduce

PUT dummy
{
    "mappings": {
        "properties": {
            "ArrField": {
                "type": "flat_object"
            }
        }
    }
}

# Succeeds
POST dummy/_doc
{
    "ArrField": {
        "ObjKey": "ObjVal"
    }
}

# Stuck forever
POST dummy/_doc
{
    "ArrField": ["ArrVal"]
}

Expected behavior

Error out due to lack of support.

Additional Details

Plugins
none

Screenshots
If applicable, add screenshots to help explain your problem.

Host/Environment (please complete the following information):

• OS: Mac
• Version: Sonoma

Additional context
Add any other context about the problem here.

[msfroh]

I just found this:

    public XContentParser parseObject() throws IOException {
        assert currentToken() == Token.START_OBJECT;
        parser.nextToken(); // Skip the outer START_OBJECT. Need to return on END_OBJECT.

        builder.startObject();

That assert is useless in production if we're not guarding this entry-point. I'll add the appropriate check.