engineering: Add BLS entry support for grub boot arg extraction

[Britel]

Summary

Adds Boot Loader Spec (BLS) support to the osmodifier grub.cfg parser. AZL4 (Fedora-based) uses BLS entries instead of inline linux commands in grub.cfg, which caused both AZL4 E2E test stages to fail.

Part 3 of the AZL4 enablement stack. Depends on PR #672 (ESP vendor-dir discovery).

Problem

extract_boot_args_from_grub_cfg() parses linux /boot/vmlinuz ... args lines from grub.cfg to sync boot args into /etc/default/grub. On AZL4, grub.cfg contains blscfg instead of inline menuentry blocks — the kernel command line lives in /boot/loader/entries/*.conf files. The parser found no linux line and bailed with "no linux line found in non-recovery menuentry."

This broke both AZL4 E2E stages:

• Rollback: "Failed to extract boot args from grub.cfg"
• Netlaunch: trident-install hit the same parser failure during install

Changes

Single file: crates/osmodifier/src/grub_cfg.rs

BLS fallback in extract_boot_args_from_grub_cfg()

When find_non_recovery_linux_lines() fails AND grub.cfg contains blscfg, fall back to reading boot args from BLS entry files.

New function extract_options_from_bls_entries()

• Scans /boot/loader/entries/*.conf
• Sorts lexically (matches grub's ordering)
• Skips rescue/recovery entries (case-insensitive title match)
• Returns the options line as a synthetic linux line for downstream parsing

Zero AZL3 impact

The BLS path only triggers when:

1. find_non_recovery_linux_lines() finds zero results, AND
2. grub.cfg content contains blscfg

AZL3 grub.cfg never contains blscfg (BLS is disabled via GRUB_ENABLE_BLSCFG=false). The fallback never fires on AZL3.

Testing

• 3 unit tests: BLS fallback, rescue-skip, empty-entries-dir
• Pipeline build 1133385: ALL 4 AZL4 stages passed (first time ever)
  • Rollback Testing AZL4: succeeded
  • Netlaunch Testing AZL4: succeeded
  • Both AZL4 image builds: succeeded
• All AZL3 stages passed (Functional, Servicing, Runtime, E2E)

PR Stack

#	Branch	PR	Description
1	azl4-1-grub-native	#642	AZL4 distro detection + GRUB update path
2	azl4-2-esp-layouts	#672	Generic EFI vendor-dir discovery + AZL4 ESP
3	azl4-2b-bls-grub	this	BLS entry support for boot arg extraction
4	azl4-5a-builder-infra	—	Builder infra + image acquisition
5	azl4-5b-image-pipeline	—	Image configs + pipeline stages
6	azl4-6-bm-test	—	BM-simulated netlaunch stage
7	azl4-7a-qcow2-rust	—	qcow2 base + offline-init
8	azl4-7b-rollback-stage	—	VM rollback test stage