test: fix vacuous is_err() assertions in add_pr_comment and update_pr

[github-actions]

Test Suite Reduction: src/safe_outputs/add_pr_comment.rs + src/safe_outputs/update_pr.rs

What was wrong

Five validation-rejection tests used bare is_err() without inspecting the error message. A silent change to the validation logic — e.g. the wrong field being checked, or a different code path reaching the error return — would have let these tests pass while the feature regressed unnoticed.

Test	Problem
add_pr_comment::test_validation_rejects_zero_pr_id	assert!(result.is_err()) — doesn't verify the guard that fired
add_pr_comment::test_validation_rejects_short_content	assert!(result.is_err()) — doesn't verify the content-length message
add_pr_comment::test_validation_rejects_repository_pipeline_command	assert!(result.is_err()) — doesn't verify the injection-detection message
add_pr_comment::test_validation_rejects_line_without_file_path	assert!(result.is_err()) — doesn't verify the cross-field constraint message
update_pr::test_validation_rejects_invalid_operation	assert!(result.is_err()) — doesn't verify the allowed-operations message

Changes

Each test now calls unwrap_err().to_string() and asserts the error string contains the key phrase that identifies which guard fired:

Test	Action	Asserted phrase
test_validation_rejects_zero_pr_id	Rewritten	"pull_request_id must be positive"
test_validation_rejects_short_content	Rewritten	"content must be at least 10 characters"
test_validation_rejects_repository_pipeline_command	Rewritten	"pipeline command"
test_validation_rejects_line_without_file_path	Rewritten	"line requires file_path"
test_validation_rejects_invalid_operation	Rewritten	"operation must be one of"

Verification

• cargo test: all tests pass ✅
• cargo clippy --all-targets --all-features: no errors ✅

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