[release/10.0] Redo GC heap size fix in heavily pinning scenarios (#127573)

Backport of #126043 to release/10.0

## Customer Impact

- [x] Customer reported
- [ ] Found internally

This change puts back the recently reverted fix for the GC heap size
regression with regions when there is heavy pinning.

It also fixes the issue due to which the change was recently reverted
in .NET 10. The issue was in `allocate_in_condemned_generations` where a
very
large plug could never be relocated into a new region because front
padding
was always added, which could lead to an infinite loop.

## Regression
- [x] Yes, introduced in .NET 10
- [ ] No

## Testing
CI tests, local testing using regression test, GC perf tests

## Risk
This PR restores a regions/heavy-pinning GC improvement that had already
been in main/.NET 10 for an extended period and was reverted only
because of one specific hang. The additional commit directly addresses
that hang by
fixing the near-region-sized plug relocation case in
allocate_in_condemned_generations, where front padding could otherwise
prevent placement and lead to an infinite loop. Since the reverted code
had substantial bake time and the only known regression was this
specific hang, now addressed in the same PR, the overall backport risk
is low.

Author: janvorli