dekaf: new fields added to the collection schema are excluded until new "Read"

[danielnelson]

Reproduction Steps:

1. Begin reading a dekaf topic with schema inference.
2. Write a document: {"name": "alice"}
3. Write a document with a new field not in the schema: {"name": "bob", "value": 42}

Expected:

Read documents as written:

{"name": "alice"}
{"name": "bob", "value": 42}

Actual:

New field is omitted from output:

{"name": "alice"}
{"name": "bob"}

Even after waiting for the spec_ttl expires and a new schema is picked up, the new field will be absent. In order to pick up the new field, you must disconnect or perform another operation that creates a new Read such as changing the read offset.