[connectors] Improved tests for Avro connectors - #6619

LGTM. The timestamp_tz_format was silently defaulting to the string format, so TIMESTAMP WITH TIME ZONE columns produced invalid Avro against timestamp-{micros,millis} schemas — good catch. Round-trip via gen_raw_parser_test covers the encoder-side fix, and the wire-format test pins down the decode direction.

Tightens Avro coverage for TIMESTAMP WITH TIME ZONE.

serializer.rs now sets with_timestamp_tz_format(MicrosSinceEpoch) — previously the timestamp-tz format was left at its default, which is a real gap on the output side that the new tests would have surfaced eventually. Worth noting this is a small behaviour fix, not just tests.

Coverage is solid:

• Round-trip tests for both timestamp-micros and timestamp-millis Avro logical types, with sub-millisecond values on the micros path so precision is actually exercised.
• test_timestamp_tz_wire_format decodes hand-built Value::TimestampMillis / Value::TimestampMicros records against hard-coded expected TimestampTz — good sanity check that the decoder isn't losing units.
• End-to-end kafka avro workload gains a timestamp with time zone column, with decimal tightened to decimal(10, 2) for stable Avro representation.

Clean commit, signed-off, no AI trailer. LGTM.