fix(core,cloudformation): unknown X-Amz-Target -> UnknownOperationException; UpdateStack GetAtt merge (bug-hunt)

[vieiralucas]

Summary

Compat + orchestration findings from the 2026-07-15 bug hunt (1.44, 1.46).

• 1.44 (compat) — a request with an unrecognized X-Amz-Target fell through
  to the apigateway catch-all and 404'd with a misleading "Stage not found".
  It's unambiguously an awsJson call, so dispatch now returns
  UnknownOperationException (400), matching AWS.
• 1.46 (orchestration) — UpdateStack rebuilt StackResource.attributes
  from the update handler's subset only, dropping create-time attributes and the
  well-known GetAtt overlay, so a 2nd deploy resolved Fn::GetAtt / Outputs to
  the literal "Logical.Attr". Create and update now share a
  merged_resource_attributes helper that layers create-time attrs → update
  handler's fresh values → well-known get_att overlay. Confirmed victim
  AWS::Pipes::Pipe (whose CreationTime is create-time-only) keeps its GetAtt
  output across an update.

Test plan

• E2E dispatch_unknown_target::unknown_x_amz_target_returns_unknown_operation_exception.
• E2E cloudformation_pipes::cfn_update_preserves_create_time_getatt_output
  (create → update → CreationTime output stays a real timestamp, not the literal).
• cargo build/clippy --all-targets/fmt clean on core, cloudformation, fakecloud-e2e.

No new public API surface (dispatch correctness + provisioner internals), so no
SDK/docs regeneration needed.

---

Summary by cubic

Fixes awsJson dispatch for unknown targets and preserves CloudFormation GetAtt values across updates. Unknown X-Amz-Target now returns 400 UnknownOperationException, and UpdateStack keeps create-time attributes so Outputs don’t degrade.

• Bug Fixes
  • Dispatch: Requests with X-Amz-Target but unknown operation now return 400 UnknownOperationException (no more API Gateway 404).
  • CloudFormation: UpdateStack merges create-time attrs → update attrs → well-known get_att overlay via a shared helper; Fn::GetAtt/Outputs stay correct (e.g., AWS::Pipes::Pipe CreationTime).

^{Written for commit aecf9db. Summary will update on new commits.}

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