[FLINK-40264][format/avro] Propagate legacyTimestampMapping into nested row converters

[kumarpritam863]

What is the purpose of the change

AvroToRowDataConverters#createConverter dispatches a nested ROW to the single-argument overload:

case ROW:
    return createRowConverter((RowType) type);

createRowConverter(RowType) defaults legacyTimestampMapping to true, so the flag is dropped for every nesting level below the top one. Since TIMESTAMP_LTZ is only supported by the non-legacy mapping, a nested TIMESTAMP_LTZ column
fails even when the non-legacy mapping was explicitly requested:

java.lang.UnsupportedOperationException: Unsupported type: TIMESTAMP_LTZ(3) NOT NULL

The failure happens while the deserialization schema is being constructed, so the job never starts. AvroSchemaConverter#convertToSchema handles the same type correctly with legacyTimestampMapping = false, and RowDataToAvroConverters
propagates the flag properly, so only the deserialization side is affected.

Introduced by FLINK-33198, first released in 1.19.0.

Brief change log

- `AvroToRowDataConverters` now passes `legacyTimestampMapping` down when building the converter for a nested `ROW`.

This is strictly a widening: the only code paths whose behaviour changes are ones that throw today.

Verifying this change

This change added tests and can be verified as follows:

- Added `AvroRowDataDeSerializationSchemaTest#testTimestampTypeNewMappingInNestedRow`, which round-trips a row holding a nested row with a `TIMESTAMP_LTZ(3)` column under the non-legacy mapping. Without the one-line fix it fails at

construction with Unsupported type: TIMESTAMP_LTZ(3) NOT NULL.
- Ran mvn clean verify for flink-formats/flink-avro and flink-formats/flink-avro-confluent-registry: 365 and 27 tests respectively, no failures, 0 checkstyle violations, spotless and ArchUnit clean, japicmp reports no incompatibility.

Does this pull request potentially affect one of the following parts:

- Dependencies (does it add or upgrade a dependency): **no**
- The public API, i.e., is any changed class annotated with `@Public(Evolving)`: **no**
- The serializers: **yes** — `AvroToRowDataConverters`, but only for nested rows under the non-legacy timestamp mapping, which cannot be constructed at all today.
- The runtime per-record code paths (performance sensitive): **no**
- Anything that affects deployment or recovery: JobManager (and its components), Checkpointing, Kubernetes/Yarn, ZooKeeper: **no**
- The S3 file system connector: **no**

Documentation

- Does this pull request introduce a new feature? **no**
- If yes, how is the feature documented? **not applicable**

Note for reviewers

This is a backport candidate: the bug is present in every release from 1.19.0 onward.

FLINK-40262, submitted separately, restructures createRowConverter into createRowConverterInternal and in doing so necessarily threads the same flag, so it fixes this symptom as a side effect but carries no regression test for it. Merging
this one first keeps the test; whichever lands second needs a trivial rebase.

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CI report:

• c03c530 Azure: FAILURE

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