test: fix vacuous is_err() assertions in create_git_tag and comment_on_work_item

[github-actions]

Test Suite Reduction: src/safe_outputs/create_git_tag.rs + src/safe_outputs/comment_on_work_item.rs

What was wrong

• test_validation_rejects_short_message (create_git_tag): bare assert!(result.is_err()) — passes even if the wrong validator fires (e.g. tag_name check running instead of message check).
• test_validation_rejects_repository_pipeline_command (create_git_tag): bare assert!(result.is_err()) — passes even if any unrelated validation rejects the input.
• test_validation_rejects_zero_work_item_id (comment_on_work_item): bare assert!(result.is_err()) — would pass even if a completely different field caused the error.
• test_validation_rejects_negative_work_item_id (comment_on_work_item): same issue — assertion says nothing about which field was rejected or why.
• test_validation_rejects_short_body (comment_on_work_item): same issue — body-length guard could regress without the test catching it.

Changes

Test	File	Action	Reason
test_validation_rejects_short_message	create_git_tag.rs	Rewritten	Bare is_err() → unwrap_err() + assert contains "message must be at least 5 characters"
test_validation_rejects_repository_pipeline_command	create_git_tag.rs	Rewritten	Bare is_err() → unwrap_err() + assert contains "pipeline command"
test_validation_rejects_zero_work_item_id	comment_on_work_item.rs	Rewritten	Bare is_err() → unwrap_err() + assert contains "work_item_id must be positive"
test_validation_rejects_negative_work_item_id	comment_on_work_item.rs	Rewritten	Bare is_err() → unwrap_err() + assert contains "work_item_id must be positive"
test_validation_rejects_short_body	comment_on_work_item.rs	Rewritten	Bare is_err() → unwrap_err() + assert contains "body must be at least 10 characters"

Verification

• cargo test: 2569+ tests pass, 0 failed ✅
• cargo clippy: no new warnings ✅

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