[Repo Assist] fix(rust-guard): replace to_lowercase() with eq_ignore_ascii_case, remove stale dead_code

[github-actions]

🤖 This is an automated pull request from Repo Assist.

Addresses the two improvements described in #3314.

Changes

1. guards/github-guard/rust-guard/src/labels/helpers.rs

Replace to_lowercase() + == in is_trusted_first_party_bot with eq_ignore_ascii_case:

Before: allocates a String on every call via to_lowercase()
After: zero-allocation byte-by-byte in-place comparison — consistent with the existing username_in_list helper (line 248) which already uses eq_ignore_ascii_case

All bot literals are pure ASCII lowercase, so the semantics are identical.

2. guards/github-guard/rust-guard/src/lib.rs

Remove six stale #[allow(dead_code)] attributes from:

• invoke_backend — public function, referenced in labels/backend.rs
• LogLevel enum — all variants used by the log helpers
• log_debug, log_info, log_warn, log_error — called throughout labels/backend.rs and labels/helpers.rs

These attributes were added during early development when the items were conditionally-dead, but the code has since grown and they're all actively used. Keeping them muffles any future dead_code warnings if a refactor accidentally removes a call site.

Root Cause

Both issues were legacy artefacts: to_lowercase() predates the eq_ignore_ascii_case pattern established by username_in_list, and the #[allow(dead_code)] attributes were never cleaned up after the code became active.

Test Status

✅ Rust tests: cargo test — 251 passed, 0 failed
⚠️ Go tests: Infrastructure issue — Go 1.25.0 required but unavailable in CI environment (network-restricted); this is not caused by these changes (Rust-only files modified)

Closes #3314

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[Copilot]

Pull request overview

This PR makes a small performance-oriented improvement in the Rust GitHub guard by switching a hot-path bot username check to a zero-allocation ASCII case-insensitive comparison, and it cleans up stale #[allow(dead_code)] annotations so future unused-code regressions are surfaced by the compiler.

Changes:

• Replaced to_lowercase()-based comparisons in is_trusted_first_party_bot with eq_ignore_ascii_case to avoid per-call heap allocation.
• Removed stale #[allow(dead_code)] attributes from invoke_backend, LogLevel, and internal log helpers.

Show a summary per file

File	Description
guards/github-guard/rust-guard/src/labels/helpers.rs	Updates trusted-bot username matching to use allocation-free ASCII case-insensitive comparisons.
guards/github-guard/rust-guard/src/lib.rs	Removes unnecessary dead_code suppression from live logging/backend helper APIs.

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• Files reviewed: 2/2 changed files
• Comments generated: 0