[Bug] UpdateItem and TransactWriteItems Update do not enforce the attribute-name size limit; message differs from the service

[LeeroyHannigan]

Summary

Follow-up to #347, which enforces the 64 KB attribute-name limit at every nesting depth on PutItem, BatchWriteItem PutRequest, TransactWriteItems Put, and ImportTable, and documents the remaining gap in docs/dynamodb-limits.md as Partial: UpdateItem and TransactWriteItems Update do not enforce the limit at any level. A client can store a 65,536 byte (or larger) attribute name through SET #n = :v with a long name in ExpressionAttributeNames, or through a map value carrying a long key, and the item is written.

Separately, the message ExtendDB returns where it does enforce the limit is not the service's.

Measured against the service (2026-09-16)

request	service response
PutItem, top-level name of 65,536 bytes	400 ValidationException: 1 validation error detected: Attribute name is too large, must be less than 65536 bytes
PutItem, nested map key of 65,536 bytes	400 ValidationException, same message
PutItem, nested map key of 65,535 bytes	200
UpdateItem SET doc = :v, value containing a 65,536 byte nested key	400 ValidationException, same message
UpdateItem SET #n = :v, #n mapped to a 65,536 byte name	400 ValidationException: 1 validation error detected: Attribute name is too large, must be less than 65536 bytes for key #n

ExtendDB on main (after #347):

request	ExtendDB response
PutItem, top-level or nested name of 65,536 bytes	400 ValidationException: One or more parameter values were invalid: Size of attribute name '...' has exceeded the maximum size limit of 65535 bytes
UpdateItem, either shape above	200, item written with the oversized name
TransactWriteItems Update, either shape	200

Expected

1. UpdateItem and TransactWriteItems Update reject an attribute name of 65,536 bytes or more at every depth: names introduced through ExpressionAttributeNames, and map keys inside any value in ExpressionAttributeValues.
2. The message is the service's: 1 validation error detected: Attribute name is too large, must be less than 65536 bytes, with the suffix for key #n when the offending name arrived through ExpressionAttributeNames under placeholder #n.
3. The existing Put-path message is changed to the same text, and the docs/dynamodb-limits.md row moves from Partial to Yes.

Where

validate_attribute_name_sizes in crates/core/src/validation/mod.rs already walks M and L values; the UpdateItem paths need to call it on every ExpressionAttributeValues entry and check every ExpressionAttributeNames value. LimitsConfig::max_attribute_name_bytes carries the limit.