Currently, when an object is mapped with `subobjects: false`, trying to ingest a document that has nested objects leads to a mapping exception.

In a couple of occasions, we've been talking about that it would be convenient if Elasticsearch could automatically flatten the fields instead of the client having to know about the `subobjects` setting. This would make `subobjects: false` more of a drop-in replacement that allows for a much easier adoption, without changes in the client.

Note that the `_source` should ideally stay untouched, just the stored fields should be flattened automatically.

Some related issues
* https://github.com/elastic/apm/issues/347
* https://github.com/elastic/elasticsearch/issues/88777
* https://github.com/elastic/package-spec/issues/349
server/src/test/java/org/elasticsearch/index/mapper/DocumentParserTests.java
""")));
            
            assertThat(e.getCause().getMessage(), containsString("field name cannot contain only whitespace: ['top..foo']"));
        });
    }
    @AwaitsFix(bugUrl = "https://github.com/elastic/elasticsearch/issues/28948")
    public void testDynamicDottedFieldEndingWithDot() throws Exception {
        dynamicTrueOrDynamicRuntimeTest(mapper -> {
            Exception e = expectThrows(DocumentParsingException.class, () -> mapper.parse(source("""
                {"top.foo.":1}
                """)));
            
            assertThat(e.getCause().getMessage(), containsString("field name cannot contain only whitespace: ['top.foo.']"));
        });
    }
    public void testDynamicFieldsStartingAndEndingWithDot() throws Exception {
        dynamicTrueOrDynamicRuntimeTest(mapper -> {
            Exception e = expectThrows(DocumentParsingException.class, () -> mapper.parse(source("""
                {"top..foo.":1}
                """)));
            
            assertThat(e.getCause().getMessage(), containsString("field name cannot contain only whitespace: ['top..foo.']"));
        });
    }
    public void testDynamicDottedFieldWithTrailingWhitespace() throws Exception {
        dynamicTrueOrDynamicRuntimeTest(mapper -> {
            Exception e = expectThrows(DocumentParsingException.class, () -> mapper.parse(source("""
                {"top. .foo":1}
                """)));