DF-25178: Adds custom subscription handling for EA specific implementations

[cl-efornaciari]

Summary

Adds a customSubscriptionMessages builder to WebSocketTransport so EAs can fully control how subscribe/unsubscribe WebSocket messages are built each background-execute cycle. This replaces the earlier batchSubscribeMessage / batchUnsubscribeMessage API with a more flexible hook that receives full subscription deltas (new, stale, and desired).

• Introduces WebSocketCustomBuilders with customSubscriptionMessages(context, subscriptions) => unknown[]
• Unifies outbound message construction behind an internal subscriptionMessageBuilder; per-subscription subscribeMessage / unsubscribeMessage builders continue to work via a default adapter
• Simplifies sendMessages to accept a single ordered message array
• Updates docs and tests for the new API

Motivation (DF-25178): Some providers need EA-specific subscription logic—e.g. batching multiple feeds into one message, custom ordering, or provider-specific payload shapes—that the fixed per-feed and batch-builder APIs could not express cleanly.

Backward compatibility: Existing adapters using subscribeMessage / unsubscribeMessage are unchanged. Only adapters that adopt customSubscriptionMessages need updates.

API

Configure either per-subscription builders or the custom builder, not both:

builders: {
  customSubscriptionMessages: (_context, subscriptions) => {
    const messages = []
    if (subscriptions.new.length > 0) {
      messages.push({
        request: 'subscribe',
        pairs: subscriptions.new.map((s) => `${s.base}/${s.quote}`),
      })
    }
    if (subscriptions.stale.length > 0) {
      messages.push({
        request: 'unsubscribe',
        pairs: subscriptions.stale.map((s) => `${s.base}/${s.quote}`),
      })
    }
    return messages
  },
}

[github-actions]

NPM Publishing labels 🏷️

🟢 This PR has valid version labels and will cause a minor bump.

[github-actions]

🚀 Successfully created version bump PR: #822