fix(otel): omit absent fields from error exception events - #901

Fixes #894

Problem

A callback or other operation that fails with only ErrorMessage or only
ErrorType produced an OTel exception event with an invented value for the
missing field: a message-only error reported exception.type = "Exception",
and a type-only error reported an empty exception.message. A synthetic
exception.stacktrace was emitted too. Consumers could not distinguish absent
metadata from generated values.

exception_events.py built a plain Exception carrier and called
Span.record_exception, which derives attributes from the carrier before
merging explicit attributes. Omitting a field from that explicit mapping cannot
remove the generated value. Core ErrorObject already preserves absent fields
as None.

Change

record_error_object_exception emits the event directly with
span.add_event("exception", attributes=..., timestamp=...), using the existing
mapping of checkpointed fields. The carrier exception, exception_from_error_object,
and _DEFAULT_MESSAGE fallback are removed.

• When a type or message is present, emit only the checkpointed fields. Explicit
  empty strings remain distinct from absent fields.
• When both type and message are absent, emit no exception event, including when
  a stacktrace is present. The OTel exception conventions
  require exception.message if exception.type is unset, and vice versa;
  exception.stacktrace alone does not satisfy that requirement. This guard
  avoids both malformed events and invented diagnostics. The span's error
  status and durable.operation.status still report the failure.
• Stop generating exception.escaped and synthetic stacktraces from the
  carrier. A stored, nonempty stacktrace is still included when an exception
  event is emitted. Removing synthetic stacktraces also addresses that symptom
  from [Bug]: Align OTel exception stacktrace emission with OpenTelemetry JS #886.
• Document partial fields, explicit empty strings, and event suppression in
  the README's status-mapping section.

Core checkpoint serialization, CallbackExternalError diagnostics, and the
normalized event timestamp are unchanged. No conformance requirements or
handlers are changed.

Tests

• tests/test_status_mapping.py checks the exact exception attributes for five
  partial-field combinations: message-only, type-only, and three combinations
  of explicit empty strings. These cover operation and failed-attempt recorders
  in both views; failed-attempt cases also cover replayed and non-replayed
  delivery. Exact attribute assertions exclude synthetic fields.
• test_error_without_type_or_message_records_no_event covers 12 combinations:
  both views, operation and attempt recorders, and absent, empty, or nonempty
  stacktraces. It asserts no exception events and retained span ERROR status.
• tests/e2e/test_partial_callback_error_int.py adds 12 callback cases through
  DurableFunctionTestRunner, context.wait_for_callback, and
  send_callback_failure. These cover message-only and type-only errors in
  both views, same-invocation and post-resume completion, and caught and uncaught
  failures. They check that the submitter step runs once, caught failures allow
  handler success, and uncaught failures preserve the failure statuses.

The former message-only fallback test is replaced by the partial-field tests.
test_failed_operation_without_type_or_message_uses_fallback is replaced by
test_error_without_type_or_message_records_no_event, which checks suppression
and retained error status. The carrier's fallback-message test is removed with
the function it tested.

Validation

At current head ad4e452a25e0c51ece143cda0f8aba7cd711c1cd:

• hatch run dev-otel:python -m pytest packages/aws-durable-execution-sdk-python-otel/tests/test_status_mapping.py -q:
  198 passed on Python 3.13.18.
• A direct comparison with earlier revision a2e9c7f2 reproduced an empty
  exception event for missing type/message with absent or empty stacktraces,
  and a stacktrace-only event for a nonempty stacktrace. The current code emits
  no event in all three cases.

Earlier validation reported passing callback e2e tests, formatting, linting,
and changed-file type checks. Its full OTel-suite run reported 12 failures in
tests/e2e/test_view_registration.py, also reproduced with the change reverted.
Those broader checks were not rerun during the current-head validation above.

Validation is local; Lambda and X-Ray behavior has not been verified.