feat(transaction): add update_column_doc to UpdateSchemaAction

[viirya]

Which issue does this PR close?

• Closes Add update_column_doc to UpdateSchemaAction (changing a column doc shouldn't reassign its field ID) #2742.

What changes are included in this PR?

UpdateSchemaAction only supported add_column / delete_column, so changing a column's doc meant delete + re-add — which assigns a fresh field ID. Because Iceberg resolves columns by field ID, that breaks reads of older data files (the column projects as all-NULL) and severs schema-evolution history. Editing a comment shouldn't do that.

This adds update_column_doc, modeled after iceberg-java's UpdateSchema.updateColumnDoc(String name, String newDoc):

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, dotted path
    .update_column_doc("legacy", None);                               // clear

• Some(doc) sets/replaces; None clears — the Option<String> equivalent of Java's String + null.
• The field ID and every other attribute are preserved.
• Nested fields use the dotted path form (e.g. "address.city"), consistent with delete_column. They work without special handling: doc updates are keyed by field ID (resolved from the name via field_by_name), and the existing schema rebuild already recurses into structs/lists/maps.
• The column must exist at commit time, else the commit fails with PreconditionFailed. When the same column is updated more than once, the last update wins.

Implementation is confined to transaction/update_schema.rs: a doc_updates field, the builder method, name→field-id resolution in commit (same pattern as delete validation), and a small resolved_doc helper applied in rebuild_field.

Are these changes tested?

Four new tests in transaction::update_schema:

• test_update_column_doc_preserves_field_id — sets a doc on a root column and asserts the doc changed and the field ID is unchanged (siblings untouched).
• test_update_column_doc_on_nested_field — sets a doc on person.name (nested), asserting the doc and the field/enclosing-struct IDs.
• test_update_column_doc_clears_doc — Some(...) then None on the same column; last-wins clears it.
• test_update_column_doc_missing_column_errors — updating a missing column fails with PreconditionFailed.

The first test fails if the doc-resolution logic is reverted (verified). Full iceberg lib suite (1375 tests) passes; clippy and rustfmt clean.

[viirya]

The CI failure is irrelevant:

Error: /home/runner/go/pkg/mod/github.com/go-git/go-git/v5@v5.13.0/plumbing/hash/hash.go:10:2: github.com/pjbgf/sha1cd@v0.3.0: read "https://proxy.golang.org/github.com/pjbgf/sha1cd/@v/v0.3.0.zip": stream error: stream ID 123; INTERNAL_ERROR; received from peer