[LeeroyHannigan]

error_response wrote every error message under a lower-case message key. Amazon DynamoDB sends Message on TransactionCanceledException and IdempotentParameterMismatchException, and message on every other error measured. botocore reads either key, but a client that reads the modeled Message member of those two errors from the raw body found nothing.

Measurement

Amazon DynamoDB, us-east-1, raw body of a TransactWriteItems whose only member is a failing ConditionCheck, 3 November 2026, twice with the same answer:

{"__type":"com.amazonaws.dynamodb.v20120810#TransactionCanceledException","CancellationReasons":[{"Code":"ConditionalCheckFailed","Message":"The conditional request failed"}],"Message":"Transaction cancelled, please refer cancellation reasons for specific reasons [ConditionalCheckFailed]"}

#760 has the 29 October bodies of both errors, and the other errors captured that day with message.

Change

• The key now comes from message_key, which picks Message for those two errors and message for every other one.
• TransactionInProgressException also models its member as Message. It was not measured and ExtendDB does not raise it, so it keeps message.
• REQ-ERR-002 in docs/design/01-requirements.md and the doc comment of error_response name the key.

The token mismatch text, which #760 also reports, is left to #526, so this pull request is part of #760 and does not close it.

Tests

• A unit test in response_tests.rs checks the top-level keys of the body error_response sends for both errors and for ValidationException, ConditionalCheckFailedException and TransactionConflictException. It sits in its own file so that it does not collide with the test module feat(core): throttle errors carry the service's ThrottlingReasons #532 adds at the end of response.rs.
• tests/test_error_message_key.py reads the raw body from botocore's before-parse event for a cancelled transaction, an idempotency token reused with different parameters, a failed condition and a validation error.

Testing

• cargo +1.97.0 fmt --all -- --check and cargo +1.97.0 clippy -p extenddb-server --all-targets -- -D warnings: clean.
• cargo +1.88.0 test --locked -p extenddb-server: 251 passed.
• tests/test_error_message_key.py: 4 passed against Amazon DynamoDB in us-east-1 and 4 passed against a sqlite-memory,dev-mode build of this branch. Against a build of main, the two Message tests fail.
• response.rs merges without conflicts with feat(core): throttle errors carry the service's ThrottlingReasons #532 and fix(dev-mode): refuse browser and admin requests for a foreign host #635.

Part of #760.