Set dont_forward on private channel updates and add tests

[Thrishalmadasu]

Fixes #1672.

Summary

• Set the dont_forward bit (message_flags bit 1) on channel_update messages for private channels.
• Log a warning when we receive a channel_update with dont_forward set for an unknown channel.
• Add tests to cover both outbound flag setting and inbound warning logging.

Details

• In ChannelManager::get_channel_update_for_unicast, we now compute:
  • message_flags = 1 | (1 << 1) for private channels (must_be_one + dont_forward).
  • message_flags = 1 for public channels (must_be_one only).
• In internal_channel_update, if we receive a channel_update for an unknown short_channel_id and the dont_forward bit is set, we log a warning suggesting the user check the tx_index passed to chain::Confirm::transactions_confirmed.

Tests

• New tests:
  • test_channel_update_dont_forward_flag
  • test_unknown_channel_update_with_dont_forward_logs_warning

Based on lightning/bolts#999: BOLT 7 dont_forward flag for SCID aliases.

[ldk-reviews-bot]

I've assigned @wpaulino as a reviewer!
I'll wait for their review and will help manage the review process.
Once they submit their review, I'll check if a second reviewer would be helpful.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 86.80%. Comparing base (8cdc86a) to head (b0883ba).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4340      +/-   ##
==========================================
+ Coverage   86.53%   86.80%   +0.26%     
==========================================
  Files         158      158              
  Lines      103190   102790     -400     
  Branches   103190   102790     -400     
==========================================
- Hits        89300    89223      -77     
+ Misses      11469    11222     -247     
+ Partials     2421     2345      -76     

Flag	Coverage Δ
fuzzing	36.20% <25.00%> (-0.87%)	⬇️
tests	86.09% <100.00%> (+0.26%)	⬆️

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