Auto-detect available memory when AvailableMemory is unset and apply MaxMemoryPercentage

[kushalShukla-web]

Which problem is this PR solving?

When AvailableMemory and MaxAlloc are not set, Refinery has no way to determine its memory limit. This makes configs drift from actual container memory. This PR implements the behavior requested in #1704.

Short description of the changes

• Auto-detect container memory (via cgroup) when both values are unset
• Apply MaxMemoryPercentage to compute an effective MaxAlloc
• Do not require AvailableMemory anymore; keep backwards compatibility
• Add basic tests for the new fallback logic

[kushalShukla-web]

cc: @seh @VinozzZ

[VinozzZ]

Thank you for working on the issue.

It looks like the approach in the current PR only supports Linux. We’d like to solve this in a more cross-platform way, ideally leveraging a shared or standard mechanism, rather than maintaining separate, custom implementations per platform.
The team is currently focused on the next minor release, so we won’t be able to take on a larger cross-platform rework right away. That said, we do want to revisit the implementation with portability in mind and would appreciate any ideas or suggestions for a more platform-agnostic solution.

[kushalShukla-web]

in that case can we use something like /proc/meminfo ?