Fix: Progress channel announcement state when remote_sigs already in DB

[Andezion]

(Fixes #8552)

Depends on #8493 (this PR is stacked on top of the P2TR dust limit fix)

Problem

When a node restarts after exchanging announcement_signatures with a peer - the remote_sigs are loaded from the database during channel_gossip_init(). But when the peer reconnects, it may not resend announcement_signatures if it already sent them before the disconnect!

This causes the channel to get stuck in CGOSSIP_WAITING_FOR_MATCHING_PEER_SIGS state forever, preventing the channel from being announced to the network.

Solution

Modified channel_gossip_channel_reestablished() in channel_gossip.c to check if we already have remote_sigs loaded from the database. If we do - we call update_gossip_state() to attempt to progress the channel gossip state machine instead of waiting forever for the peer to resend signatures
Added comprehensive tests in test_gossip_announcement.py:

• test_channel_announcement_after_restart_with_saved_sigs: Reproduces the exact scenario from issue A specific channel is not announcing my side #8552 (channel announcement after node restart)

• test_channel_announcement_reconnect_without_restart: Verifies channel announcement works after simple reconnect without restart

Important

26.04 FREEZE March 11th: Non-bugfix PRs not ready by this date will wait for 26.06.

RC1 is scheduled on March 23rd

The final release is scheduled for April 15th.

Checklist

Before submitting the PR, ensure the following tasks are completed. If an item is not applicable to your PR, please mark it as checked:

• The changelog has been updated in the relevant commit(s) according to the guidelines.
• Tests have been added or modified to reflect the changes.
• Documentation has been reviewed and updated as needed.
• Related issues have been listed and linked, including any that this PR closes.
• Important All PRs must consider how to reverse any persistent changes for tools/lightning-downgrade