fix: recompute eviction threshold for discovered memory

[maxsxu]

Fixes #9455

Description

• recompute the hard memory eviction threshold whenever discovered memory capacity replaces the fallback instance-type capacity
• preserve absolute and default memory eviction thresholds while leaving ephemeral-storage calculations unchanged
• cover percentage, absolute, and default thresholds and verify the resulting allocatable memory

How was this change tested?

• go test ./pkg/controllers/providers/instancetype/capacity ./pkg/providers/instancetype
• make presubmit

Does this change impact docs?

• Yes, PR includes docs updates
• Yes, issue opened: #
• No

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.