[Repo Assist] refactor(rust-guard): extract extract_items_slice and is_pr_merged helpers

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Closes #8409

Problem

Issue #8409 identified two duplication patterns in the Rust guard:

1. 26-line duplicate item-extraction block (response_items.rs)

The PR arm and issue arm both contained identical logic to build a &[Value] slice from a response, trying root array → items envelope → type-specific envelope → GraphQL nodes → GraphQL single object → bare REST singleton. The only difference was the fallback field name ("pull_requests" vs "issues"). Any future change to the extraction logic required a two-place update.

2. 4-line duplicate merged-status check (helpers.rs + backend.rs)

pr_integrity in helpers.rs and get_pull_request_facts_with_callback in backend.rs both duplicated the same merged_at/merged boolean check. backend.rs additionally used raw string literals instead of the field_names constants used everywhere else, creating a drift risk.

Fix

response_items.rs: Extract a private extract_items_slice<'a>(response, list_field) helper. Each call site collapses from 26 lines to one:

// PR arm:
let items = extract_items_slice(&actual_response, "pull_requests");

// Issue arm:
let items = extract_items_slice(&actual_response, "issues");

helpers.rs: Add pub(crate) fn is_pr_merged(item: &Value) -> bool using the field_names constants. Update pr_integrity to delegate to it.

backend.rs: Import and use is_pr_merged. Removes the raw "merged_at"/"merged" string literals.

Impact

• Net -20 lines of code
• Single authoritative extraction path for all multi-shape PR/issue responses
• Single authoritative merged-status check using field_names constants throughout
• No behaviour change — same extraction logic, same field ordering

Test Status

All 556 Rust unit tests pass (cargo test green). Go tests skipped due to sandbox network restrictions (infrastructure-only, not caused by these changes).

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Pull request overview

This PR refactors the Rust GitHub guard labeling code to remove duplicated logic when (1) extracting item slices from varied response shapes and (2) determining whether a pull request is merged, aligning with the duplication report in #8409.

Changes:

• Extracted a shared extract_items_slice helper to centralize multi-shape response item extraction for both PR and issue labeling paths.
• Added a shared is_pr_merged helper (using existing field_names constants) and updated both pr_integrity and the backend PR-facts fetcher to use it.
• Removed duplicated inlined extraction/merged-check logic from call sites, reducing drift risk.

Show a summary per file

File	Description
guards/github-guard/rust-guard/src/labels/response_items.rs	Replaces duplicated item-slice extraction logic in PR/issue arms with a single extract_items_slice helper.
guards/github-guard/rust-guard/src/labels/helpers.rs	Introduces is_pr_merged helper and updates pr_integrity to delegate to it.
guards/github-guard/rust-guard/src/labels/backend.rs	Uses the shared is_pr_merged helper instead of duplicated merged-status checks with raw string keys.

Review details

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• Files reviewed: 3/3 changed files
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• Review effort level: Low