test: fix vacuous assertions in src/execute.rs

[github-actions]

Test Suite Reduction: src/execute.rs (round 2)

What was wrong

Five malformed-input tests only asserted result.is_err() with no check on the error message. These are vacuous from a regression-detection standpoint: the assertion passes regardless of why the function failed. If the implementation changed to reject inputs with an unrelated error (e.g. a panic, a network timeout, or wrong dispatch), the test would still pass silently.

Changes

Test	Action	Reason
test_execute_safe_output_malformed_work_item_returns_err	Rewritten	Bare is_err() — upgraded to unwrap_err() + "Failed to parse create-work-item" + "missing field" checks
test_execute_malformed_update_wiki_page_returns_err	Rewritten	Bare is_err() — same pattern
test_execute_malformed_create_wiki_page_returns_err	Rewritten	Bare is_err() — same pattern
test_execute_malformed_upload_pipeline_artifact_returns_err	Rewritten	Bare is_err() — same pattern
test_execute_malformed_comment_on_work_item_returns_err	Rewritten	Bare is_err() — same pattern

Each rewrite checks:

1. The error message contains "Failed to parse {tool-name}" — confirming the correct dispatch path is hit (not a random earlier failure).
2. The error message contains "missing field" — confirming serde deserialization is the actual failure mechanism.

Verification

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

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