feat: add support for broadcast replay configuration

[grdsdev]

Summary

• Added ReplayOption class for configuring broadcast replay
• Added replay field to RealtimeChannelConfig
• Updated toMap() method to include replay configuration in broadcast config

Changes

This PR implements broadcast replay support following the pattern from PR supabase/realtime-js#540:

1. ReplayOption: New class with since (int) and optional limit (int?)
2. RealtimeChannelConfig: Added optional replay field
3. toMap(): Updated to serialize replay configuration

Usage

// Configure broadcast with replay
final channel = supabase.channel(
  'my-channel',
  RealtimeChannelConfig(
    self: true,
    ack: true,
    replay: ReplayOption(
      since: 1234567890,
      limit: 100,
    ),
  ),
);

// Broadcast callback receives meta field
channel.onBroadcast(
  event: 'position',
  callback: (payload) {
    final meta = payload['meta'] as Map<String, dynamic>?;
    if (meta?['replayed'] == true) {
      print('Replayed message: ${meta?['id']}');
    }
  },
).subscribe();

Test plan

• Verify type checking passes
• Test broadcast replay configuration
• Verify meta field is properly included in callbacks

🤖 Generated with Claude Code

[coveralls]

Pull Request Test Coverage Report for Build 18144753535

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This pull request's base commit is no longer the HEAD commit of its target branch. This means it includes changes from outside the original pull request, including, potentially, unrelated coverage changes.

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Details

• 4 of 10 (40.0%) changed or added relevant lines in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage decreased (-0.1%) to 80.477%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
packages/realtime_client/lib/src/types.dart	4	10	40.0%

Totals
Change from base Build 18131530215:	-0.1%
Covered Lines:	3137
Relevant Lines:	3898

---

💛 - Coveralls

[edgurgel]

LGTM

[Vinzent03]

If I'm correct, your usage example is not entirely correct as the js documentation states that it only works for private channels. I think this should be added to the documentation of the ReplayOption as well. The limit of 25 and it only works for messages sent from the database should be added as well. Basically this block from the js readme:

Broadcast Replay enables private channels to access messages that were sent earlier. Only messages published via Broadcast From the Database are available for replay.