fix(pubsub): increase subscription buffer size to 256 and update go-libp2p-pubsub

[olegshmuelov]

1. Increase Subscription Buffer Size to 256:
   • This is a follow-up to PR #1989, where the subscription buffer size was increased to 128. That change significantly reduced message drops and improved performance.
   • However, a small number of "Can't deliver message to subscription" logs persisted during high traffic bursts. Increasing the buffer size to 256 provides additional capacity to handle message bursts more effectively and further reduces the likelihood of drops.
2. Update go-libp2p-pubsub to v0.12.0:

[codecov]

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All modified and coverable lines are covered by tests ✅

Project coverage is 48.1%. Comparing base (4e1c82f) to head (fc3b373).
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[moshe-blox]

Great performance boost 💪