dekaf: incompatible field schema closes session until spec refresh

[danielnelson]

Reproduction Steps:

1. Begin reading a dekaf topic with schema inference.
2. Write a document: {"name": "alice", "value": 42}
3. Write a document with an incompatible field schema: {"name": "bob", "value": "howdy"}

Expected:

Read documents as written due to schema widening:

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

Actual:

Disconnected:

%6|1786407316.978|FAIL|rdkafka#consumer-1| [thrd:sasl_plaintext://127.0.0.1:10250/1]: sasl_plaintext://127.0.0.1:10250/1: Disconnected (after 21858ms in state UP)
%6|1786407317.015|FAIL|rdkafka#consumer-1| [thrd:sasl_plaintext://127.0.0.1:10250/1]: sasl_plaintext://127.0.0.1:10250/1: Disconnected (after 12ms in state UP, 1 identical error(s) suppressed)
% ERROR: Local: Broker transport failure: sasl_plaintext://127.0.0.1:10250/1: Disconnected (after 21858ms in state UP)
% ERROR: Local: All broker connections are down: 1/1 brokers are down: terminating

Server logs:

2026-08-11T00:15:17.014501Z  WARN dekaf_session{session_client_id="rdkafka" task_name="dbn/test/dekaf-generic"}:serve{idle_timeout=60s stale_read_timeout=60s tls_handshake_timeout=10s addr=[::1]:50252}: dekaf: error=Extracting field Extractor(
    Extractor {
        ptr: Pointer(
            [
                Property(
                    "value",
                ),
            ],
        ),
        policy: SerPolicy {
            str_truncate_after: 18446744073709551615,
            array_truncate_after: 18446744073709551615,
            nested_obj_truncate_after: 18446744073709551615,
        },
        default: Null,
        magic: None,
    },
), schema: Long
Caused by:
    at , value "howdy" does not conform to AVRO schema "long"

Cannot read again until spec_ttl timeout refreshes the collection spec.