feat: restrict response sizes to expected sizes

[mrzeszutko]

P2P ReqResp: Restrict response sizes to expected sizes

Summary

P2P request responses were previously limited to a default max size of 10MB for all ReqResp protocols. This change makes the limits dynamic based on what was actually requested. For example, if requesting 8 transactions, the limit is now 8 × MAX_TX_SIZE_KB + 1 KB instead of a blanket 10MB.

This reduces the attack surface for oversized response DoS and ensures predictable memory usage.

Changes

Size calculation per protocol

Protocol	Request Contains	Max Response Size
TX	TxHashArray	count × 512 KB + 1 KB
BLOCK_TXS	BitVector	requestedCount × 512 KB + 1 KB
BLOCK	Fr (block number)	Fixed 3 MB (TxEffects only, no proofs)
STATUS	StatusMessage	1 KB
PING	minimal	1 KB
AUTH	AuthRequest	1 KB
GOODBYE	minimal	1 KB

Files changed

• stdlib/src/p2p/constants.ts — Added MAX_L2_BLOCK_SIZE_KB constant (3 MB)
• p2p/src/services/encoding.ts — Added maxSizeKbOverride parameter to inboundTransformData() so callers can override topic-based limits
• p2p/src/services/reqresp/protocols/tx.ts — Added calculateTxResponseSize() that computes expected size from TxHashArray length
• p2p/src/services/reqresp/protocols/block_txs/block_txs_reqresp.ts — Added calculateBlockTxsResponseSize() that computes expected size from BitVector indices
• p2p/src/services/reqresp/interface.ts — Added subProtocolSizeCalculators map linking each protocol to its size calculator
• p2p/src/services/reqresp/reqresp.ts — sendRequestToPeer() now computes expected response size from request payload and passes it through to decompression validation

Tests added

• protocols/tx.test.ts (new) — Unit tests for calculateTxResponseSize covering single hash, multiple hashes, batch size, raw hash fallback, garbage input, and empty array
• protocols/block_txs/block_txs.test.ts — Unit tests for calculateBlockTxsResponseSize covering various BitVector configurations and error cases
• encoding.test.ts — Tests for maxSizeKbOverride parameter precedence over topic and default limits

Notes

• Gossip sub topic limits (block_proposal, checkpoint_proposal, etc.) are not changed in this PR — only ReqResp protocols
• The existing MAX_TX_SIZE_KB (512 KB) constant is reused for all transaction size calculations
• Size calculators gracefully handle unparseable request buffers by falling back to a single transaction size limit

Resolves A-469

[AztecBot]

Flakey Tests

🤖 says: This CI run detected 1 tests that failed, but were tolerated due to a .test_patterns.yml entry.

\033FLAKED\033 (8;;http://ci.aztec-labs.com/d6e889bfead37005�d6e889bfead370058;;�):  yarn-project/end-to-end/scripts/run_test.sh simple src/e2e_epochs/epochs_mbps.parallel.test.ts "builds multiple blocks per slot with transactions anchored to proposed blocks" (231s) (code: 1) group:e2e-p2p-epoch-flakes

[mrzeszutko]

@PhilWindle merging - just rebased on merge-train/spartan to fix CI issues