Improve BFT resilience of BlockBuffer stores

[jordanschalm]

This PR refactors the PendingBlockBuffer used by compliance engines to store disconnected blocks.

• Consolidates Proposal types using generics (similar to Block)
• Consolidates PendingBlockBuffer using generics, so we have one type-safe implementation
• Changes the backend of PendingBlockBuffer to use LevelledForest instead of bespoke backend
  • We remove the DropForParentID method, because LevelledForest has no parallel of this (it's a very minor memory optimization, and has the potential to drop unprocessed blocks in a concurrent environment)
• Adds a ActiveViewRangeSize to the PendingBlockBuffer, which puts an upper limit on what views we will accept when adding blocks.
  • The compliance engine does already have a similar mechanism (SkipNewProposalsThreshold), however including it at the PendingBlockBuffer layer as well is inexpensive and provides a more isolated guarantee that the memory usage is bounded, so I was inclined to include it here as well.

See #8170 for detail.

NOTE: Some of these changes (primarily the test suite) were done with AI (Cursor CLI - Composer 1). I marked commits which were substantially authored by AI with an ai: prefix (just so you are aware -- I have reviewed them 😃).

[github-actions]

Dependency Review

✅ No vulnerabilities or license issues or OpenSSF Scorecard issues found.

Scanned Files

None

[codecov-commenter]

Codecov Report

❌ Patch coverage is 27.77778% with 169 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
module/mock/generic_pending_block_buffer.go	0.00%	70 Missing ⚠️
module/mock/buffered_proposal.go	0.00%	16 Missing ⚠️
engine/collection/compliance/core.go	22.22%	10 Missing and 4 partials ⚠️
engine/consensus/compliance/core.go	22.22%	10 Missing and 4 partials ⚠️
module/mock/pending_block_buffer.go	0.00%	14 Missing ⚠️
module/mock/pending_cluster_block_buffer.go	0.00%	14 Missing ⚠️
module/mempool/errors.go	0.00%	10 Missing ⚠️
module/buffer/pending_blocks.go	91.80%	5 Missing ⚠️
engine/collection/epochmgr/factories/compliance.go	0.00%	4 Missing ⚠️
engine/collection/compliance/engine.go	0.00%	1 Missing and 1 partial ⚠️
... and 4 more

📢 Thoughts on this report? Let us know!

[durkmurder]

How about caching the header to avoid overhead of calling the ProposalHeader() and additional functions in each call?

[durkmurder]

🏅 This is great, I like it a lot, a clean implementation and a very useful concept.