Kafka Connect: iceberg.tables.schema-force-optional only applies to top-level columns — nested struct fields are still created as required

### Apache Iceberg version

1.11.0 (also reproduced on a recent 1.12.0-SNAPSHOT build)

### Query engine

Kafka Connect

### Please describe the bug 🐞

With `iceberg.tables.schema-force-optional=true` and auto-create enabled, the sink creates top-level columns as optional (as documented), but fields **nested inside a struct column** are still created as `required` when the incoming Connect schema marks them required.

Concrete case: a CDC pipeline using an SMT that attaches a metadata struct to each record (e.g. the bundled `DebeziumTransform`, whose `_cdc.key` struct carries the source table's primary-key fields). On auto-create, the struct's child fields inherit the Connect schema's `required`, so every table is born with required nested fields even though the operator asked for force-optional.

Why this matters: if the source table's primary key later changes (`ALTER TABLE ... DROP/ADD PRIMARY KEY`), the old key fields disappear from incoming records, but the Iceberg schema still requires them inside the struct. The writer then fails on every record for that table (an NPE in the writer path in our case), and because this happens inside `put()`, `errors.tolerance=all` / DLQ does not catch it — the task crash-loops and the whole connector's throughput stalls. We measured a multi-day bronze-ingest outage for one source before diagnosing; the fleet-wide fix was making ~16K nested key fields optional by hand with metadata-only schema commits.

Expected behavior: `schema-force-optional=true` applies recursively — every field at every nesting level created (or evolved in) by the sink should be optional, so a shrinking source schema can never make a table unwritable.

Happy to test a fix against a busy CDC deployment (thousands of auto-created tables), and to propose a patch if maintainers agree the recursive interpretation is the intended semantics.

### Willingness to contribute

- [x] I can contribute a fix for this bug independently
- [x] I would be willing to contribute a fix for this bug with guidance from the Iceberg community
- [ ] I cannot contribute a fix for this bug at this time

Trace the Kafka Connect sink handling of schema-force-optional from configuration through auto-created nested struct schemas, starting with the nested required-field case described for DebeziumTransform. Reproduce a record with required fields inside a struct and verify that creation or schema evolution makes every nested field optional and that a later shrinking source schema remains writable.

Written by the indexing model from the issue text.