refactor: reduce complexity of execute_impl in close_github_issue.rs

[github-actions]

What was complex

CloseGithubIssueResult::execute_impl in src/safe_outputs/close_github_issue.rs was flagged by clippy::too_many_lines at 240/100 lines. The single function handled: config/token validation, target resolution, duplicate-target resolution and node-ID lookup, the already-closed short-circuit, posting an optional closing comment, the PATCH to close the issue, the GraphQL markAsDuplicate mutation, and building the final success message/JSON payload.

What changed

Extracted five well-named helper functions, each owning one distinct responsibility:

• resolve_canonical_duplicate — resolves and validates the duplicate_of target (repository/self-reference checks, capability check, GraphQL node-ID lookup), returning Option<DuplicateCanonical>.
• post_closing_comment — posts the optional closing comment (no-op if already closed / omitted / no body).
• close_issue — sends the PATCH that transitions the issue to closed (no-op if already closed).
• mark_as_duplicate — runs the markAsDuplicate GraphQL mutation and folds failures into an ExecutionResult that still reports the issue as closed.
• build_success_result — builds the final message and JSON payload for the success path.

A small DuplicateCanonical struct replaces an inline 4-tuple to make the duplicate-resolution result self-documenting.

No public API or behavior changes — all control flow, error messages, and JSON payload shapes are preserved exactly.

Before / after

• Before: this function has too many lines (240/100)
• After: this function has too many lines (110/100)

Verification

• cargo test --bin ado-aw — 3229 passed, 0 failed, 1 ignored (full suite, including all 14 close_github_issue tests unchanged).
• cargo clippy --all-targets --all-features — clean for this file (one pre-existing, unrelated warning in create_work_item.rs).

Warning

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Generated by Cyclomatic Complexity Reducer · auto · 116.8 AIC · ⌖ 17.5 AIC · ⊞ 11.4K · ◷

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