feat(RequestBuffer): throttle prefetch hints based on MSHR pressure

[ywlcode]

Summary

This PR adds MSHR-pressure-based throttling for A-channel prefetch hints in RequestBuffer.

When MSHR and s2/s3 pipeline occupancy approach the allocation limit, A-channel Hint requests are blocked
from:

• flowing directly through RequestBuffer
• allocating into RequestBuffer
• issuing from existing RequestBuffer entries
• already in the chosenQ

This prevents prefetch hints from consuming scarce MSHR resources when the L2 is close to saturation, leaving
more room for demand req.

Changes

• Add s2/s3 pipeline status inputs to RequestBuffer.
• Compute prefetch blocking based on valid MSHR entries plus valid s2/s3 pipeline requests.
• Gate only A-channel demand requests under high MSHR pressure.

Performance