fix(etcd): fail role probe on status errors

[weicao]

Problem

roleprobe.sh returned exit code 0 with empty stdout when etcdctl endpoint status failed because the top-level command substitution did not propagate get_etcd_role failure. A successful but malformed endpoint response could also produce a false role.

Change

• propagate get_etcd_role failure from the script entrypoint
• reject endpoint output missing, malformed, or duplicate MemberID, Leader, or IsLearner fields
• add focused failure fixtures for endpoint errors, malformed output, duplicate fields, and entrypoint rc propagation

Verification

• shellspec --load-path ./shellspec --shell /opt/homebrew/bin/bash addons/etcd/scripts-ut-spec/roleprobe_spec.sh: 19 examples, 0 failures
• shellspec --load-path ./shellspec --default-path addons/etcd/scripts-ut-spec --shell /opt/homebrew/bin/bash: 84 examples, 0 failures
• bash -n addons/etcd/scripts/roleprobe.sh
• shellcheck addons/etcd/scripts/roleprobe.sh
• git diff --check 79c82e3ba6adb532625295a78046b82edd3a2d90..HEAD

This PR is intentionally stacked on exact candidate head 79c82e3ba6adb532625295a78046b82edd3a2d90 and does not change the peer-network/TLS topology path.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 0% with 144 lines in your changes missing coverage. Please review.
✅ Project coverage is 0.00%. Comparing base (79c82e3) to head (276a9b4).

Files with missing lines	Patch %	Lines
addons/etcd/scripts-ut-spec/roleprobe_spec.sh	0.00%	144 Missing ⚠️

Additional details and impacted files

@@                        Coverage Diff                         @@
##           martin/etcd-memberjoin-idempotency   #3267   +/-   ##
==================================================================
  Coverage                                0.00%   0.00%           
==================================================================
  Files                                     150     150           
  Lines                                   23843   23932   +89     
==================================================================
- Misses                                  23843   23932   +89     

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