fix(gossipsub): allow configuring max outbound message size

[paschal533]

Problem

Publishing a message larger than 4 MiB via pubsub.publish() fails with an opaque error thrown deep inside the length-prefixed encoder:

InvalidDataLengthError: Message length too long
    at validateMaxDataLength (it-length-prefixed/encode.ts)
    at encode.single (it-length-prefixed/encode.ts)
    at OutboundStream.push (gossipsub/stream.ts)
    at GossipSub.sendRpc (gossipsub/gossipsub.ts)
    at GossipSub.publish (gossipsub/gossipsub.ts)

The it-length-prefixed encoder has a hardcoded 4 MiB default (MAX_DATA_LENGTH). The gossipsub outbound path never passed options to override it — unlike the inbound path which already exposes maxInboundDataLength. There was no way to raise the limit.

Closes #3427

Changes

src/index.ts

Add maxOutboundDataLength?: number to GossipsubOpts, mirroring the existing maxInboundDataLength option.

src/stream.ts

Add maxDataLength? to OutboundStreamOpts, store it on the OutboundStream instance, and pass it to encode.single() so the encoder uses the configured value instead of the library default.

src/gossipsub.ts

Three changes:

• createOutboundStream() — pass maxDataLength: this.opts.maxOutboundDataLength when constructing OutboundStream
• sendRpcInBatch() — pass { maxDataLength: this.opts.maxOutboundDataLength } to its own encode.single() call (previously uncovered)
• publish() — add an early guard that fires before any protobuf encoding or network I/O, throwing with a clear, actionable message:

Message is too large (6291456 bytes). The current limit is 4194304 bytes.
Increase the maxOutboundDataLength option to send larger messages.

test/message-size.spec.ts (new)

Four test cases:

1. Oversized message with default config rejects with an actionable error
2. Message exactly at the 4 MiB default limit passes
3. Configuring maxOutboundDataLength: 8 MiB allows a 6 MiB message through
4. Error message includes actual size, limit, and option name

User fix

libp2p.services.pubsub = gossipsub({
  maxOutboundDataLength: 8 * 1024 * 1024  // raise to 8 MiB
})

Test plan

• All 4 new tests pass
• Full gossipsub node test suite passes (97 tests)
• Verified live with a demo script: scenario 1 rejects clearly, scenario 2 publishes successfully to a peer, scenario 3 passes at the exact boundary