Enhance GitHub Actions CI test coverage workflow and expand load_guard unit tests

[aghassemlouei]

Summary of Changes

This PR improves GitHub Actions CI test coverage enforcement for changes going into main, and adds comprehensive unit test coverage across multiple core load_guard modules.

1. GitHub Actions CI Enhancements (.github/workflows/test.yml)

• Expanded Branch Triggers: Updated push and pull_request triggers so CI automatically runs on main as well as feature, fix, and doc branches ('feature/**', 'feat/**', 'fix/**', 'docs/**'), ensuring automated test coverage validation on all changes going into main.
• Documentation Tests: Added cargo test --workspace --doc --verbose to the test job so doc tests are executed in CI alongside unit and integration tests.
• Workspace-Wide Coverage: Added the --workspace flag to cargo llvm-cov to ensure coverage metrics are gathered across all workspace targets and members.

2. Expanded Unit Tests in src/load_guard/

• src/load_guard/route.rs: Added unit tests for is_tunnel_interface covering tunnel interface prefixes (utun0, tun1, ppp0, ipsec0) versus physical/local interfaces (en0, en1, eth0, wlan0, lo0), as well as edge cases for parse_default_route (missing interface line, empty output).
• src/load_guard/budget.rs: Added unit tests verifying DurationExceedsCap and ConcurrencyExceedsCap validation errors, plus tests for BudgetError::Display formatting across all error variants.
• src/load_guard/counters.rs: Added unit tests verifying InterfaceCounters::zero() initialization, usable_delta_from across equal/increasing deltas and backwards tx/rx byte/packet deltas, and parse_netstat_ib handling of empty strings and malformed headers.
• src/load_guard/tcp_vs_udp.rs: Added unit tests verifying achieved_mbps_delta() calculations when both protocols are usable, and validated unusable helper field assignments.
• src/load_guard/ap_identity.rs: Added unit tests for compare with None bands and differing/identical labels, verified JSON serialization/deserialization roundtrip for ApIdentity and ApComparison, and added edge case tests for empty inputs to label_for_bssid.