#21799)

## Motivation

The contract instance store used `.set()` which silently overwrote
existing entries, while the contract class store used
`.setIfNotExists()` which silently ignored duplicates. Neither behavior
catches the unexpected case of a double-add, which could lead to data
loss on rollback — if an instance is added at block N and again at block
M, rolling back block M would delete the instance entirely, invalidating
the first add. The protocol prevents this via deployer nullifiers, but
the store should enforce it as defense-in-depth.

## Approach

Both `addContractInstance` and `addContractClass` now check for existing
entries and throw if the key already exists. This surfaces any
unexpected double-adds as errors rather than silently corrupting state.

## Changes

- **archiver (contract_instance_store)**: `addContractInstance` checks
`hasAsync` before writing; throws with a descriptive error on duplicate
- **archiver (contract_class_store)**: `addContractClass` replaces
`setIfNotExists` with explicit `hasAsync` check and throw on duplicate
- **archiver (tests)**: Updated "add twice" tests to expect throws
instead of silent success

BEGIN_COMMIT_OVERRIDE
feat(p2p): add tx validator for contract instance deployment addresses
(#21771)
fix: always deploy IRM for testnet (#21755)
fix: avoid mutating caller's array via splice in snapshot sync (A-718)
(#21759)
chore: update network logs skill (#21785)
feat(archiver): validate contract instance addresses before storing
(#21787)
fix: ensure no division by 0 (#21786)
feat: support private fork releases via ci-release (#21778)
fix: restrict scenario deployments to only nightly (#21798)
fix(stdlib): zero-pad bufferFromFields when declared length exceeds
payload (#21802)
test(protocol-contracts): verify max-size bytecode fits in contract
class log (#21818)
fix: wire BOT_DA_GAS_LIMIT through helm/terraform for staging-public
(#21809)
fix: remove jest-mock-extended from worker processes + fix
parallelize_strict silent failures (#21821)
fix(archiver): throw on duplicate contract class or instance additions
(#21799)
chore: remove broadcasted function events (#21805)
fix: sync dateProvider from anvil stdout on every mined block (#21829)
fix(sequencer): use wall-clock time instead of L1 block timestamp for
slot estimation (#21769)
fix: use correct EthCheatCodes method name in epochs_missed_l1_slot test
(#21848)
feat(p2p): add tx validator for contract class id verification (#21788)
feat: publisher funding (#21631)
feat: batch chonk verifier TS integration (#21823)
fix(sequencer): remove l1 block timestamp check (#21853)
fix: use local IVC inputs for batch_verifier bench test (#21857)
fix(p2p): centralize gossipsub penalization and fix inconsistencies
(#21863)
chore: publish GitHub releases to AztecProtocol/barretenberg (#21775)
END_COMMIT_OVERRIDE

BEGIN_COMMIT_OVERRIDE
chore: backport #21754 (feat!: make isContractInitialized a tri-state
enum) to v4-next (#21792)
fix(stdlib): zero-pad bufferFromFields when declared length exceeds
payload (#21802)
test(protocol-contracts): verify max-size bytecode fits in contract
class log (#21818)
chore: port P2P mesh topic deflake fix to v4-next (#21825)
fix(archiver): throw on duplicate contract class or instance additions
(#21799)
feat: sync poseidon in the browser (#21833)
chore: backport #21824 (fix(aztec-up): add sensible defaults to
installer y/n prompts) to v4-next (#21844)
fix(sequencer): backport wall-clock time for slot estimation to v4-next
(#21769) (#21847)
chore: backport PR #21788 (feat(p2p): add tx validation for contract
class id verification) to v4-next (#21852)
feat: sync poseidon browser (#21851)
END_COMMIT_OVERRIDE