Add update_column_doc to UpdateSchemaAction (changing a column doc shouldn't reassign its field ID)

[viirya]

Is your feature request related to a problem or challenge?

UpdateSchemaAction currently only supports add_column and delete_column. There is no way to change just the documentation string (doc) of an existing column.

To change a column's doc today, the only option is to delete and re-add the column. That assigns a fresh field ID to the "new" column (add_column always allocates a new id from last_column_id). Since Iceberg identifies columns by field ID, changing the id breaks reads of older data files written under the old id (the column projects as all-NULL) and severs the column's schema-evolution history. So a harmless "edit the comment" operation silently corrupts data.

Describe the solution you'd like

Add UpdateSchemaAction::update_column_doc, mirroring iceberg-java's UpdateSchema.updateColumnDoc(String name, String newDoc), which updates only the doc and preserves the field ID and every other attribute.

tx.update_schema()
    .update_column_doc("z", Some("the z column".to_string()))   // set/replace
    .update_column_doc("person.name", Some("full name".to_string())) // nested via dotted path
    .update_column_doc("legacy", None);                          // clear the doc

• Some(doc) sets/replaces the doc; None clears it. This is the Option<String> equivalent of Java's String + null argument.
• Nested fields are addressed by dotted path (e.g. "address.city"), consistent with delete_column.
• The column must exist in the current schema at commit time, otherwise the commit fails with DataInvalid/PreconditionFailed.

Willingness to contribute

I can contribute this independently.

[TwinklerG]

The feature has already been implemented in #2451.

[viirya]

Thanks for the pointer @TwinklerG — confirmed, #2451 covers this (UpdateColumn::new_doc, applied via UpdateColumnOperation::Doc in schema_update, with a dedicated update_doc_preserves_other_metadata test). It's part of a much larger UpdateSchemaAction refactor there (required/optional toggle, type promotion, doc, default value, column move), so it supersedes this narrower PR rather than just overlapping it.

Closing #2743 in favor of #2451.