Fix MessagePack serialization on Node 24

[holmesworcester]

Pin all four auth workspace consumers to MessagePack 1.11.8 and exercise long Unicode, nested and binary payloads through the real crypto APIs. This is the companion dependency update for TryQuiet/quiet#3423's embedded Node 24 migration.

The original failure was MessagePack 1.10.2 passing an out-of-bounds Buffer write length on Node 24. Current main already uses 1.11.2 and passes the baseline; this refresh preserves all current security fixes and retains the migration's tested 1.11.8 pin. Transitive relay dependencies remain unchanged.

Six regression tests cover serialization, hashing, signatures and tampering, symmetric encryption, and provided/ephemeral asymmetric sender keys. Signature coverage uses the current required domain-separation context and also rejects verification under another context. No crypto algorithm or wire-protocol implementation changes.

CI uses frozen pnpm 10.6.0 installs and runs the full suite on Node 20 and 24.

Fresh validation at 17e0b5b (2026-09-21):

• Frozen installs and complete workspace builds pass on Node 20.20.1 and Node 24.13.0.
• Full test suite: 838 passed, zero failures or skips on each Node version.
• Isolated unmodified current-main baseline on Node 24: 832 passed, accounting for the six new tests.

This supersedes the earlier pre-refresh results that reported 88 failures on an older auth baseline. Fresh hosted CI is pending.