feat(discovery): report available content pack versions in metadata

[mayankpande88]

Description

discovery_inventory requires the caller to pin content_pack_version — there is no "latest" default, by design (packs are locally cached, signature-verified files). But nothing told the server which versions an agent actually has, so a server-side caller would have to hardcode a version and find out at call time whether it exists.

CollectMetadata now includes a pack_versions field: the linux-inventory-v<N>.yaml versions present in pack_dir, ascending. This rides the existing datasource_metadata message, which the relay already persists into integrations.labels, so the server can pick a version to pin instead of guessing. The key is absent when no pack_dir is configured — same as an older agent that predates the field. Presence means the file exists; signature verification still happens at execution time, unchanged.

Server-side consumer context: nudgebee/nudgebee-enterprise#35881 (discovery datasource registration) and the scan pipeline in nudgebee/nudgebee-enterprise#35822.

Part of #116.

Type of change

• Enhancement (non-breaking change which improves existing functionality)

How Has This Been Tested?

• Unit tests: version listing (ordering, non-matching files ignored) and key absence without pack_dir
• Manual testing

Checklist

• CLA signed (the CLA bot will prompt on your first PR)
• make validate passes (fmt + lint + test)
• Docs updated if the wire shape, config surface, or proxy module behavior changed — the new field is optional and documented at the source; docs/proxy-modules.md has no discovery section yet (pre-existing gap, not extended here)

[gemini-code-assist]

Code Review

This pull request implements content pack version reporting in the discovery proxy metadata. It scans the configured pack_dir for files matching linux-inventory-v<N>.yaml, extracts and sorts the version numbers, and includes them in the collected metadata. A test suite has also been added to verify this behavior. The review feedback suggests ignoring 'not exist' errors when reading pack_dir to prevent misleading warning logs on fresh installations before content packs are synced.