internal/rest/resources: Trust a joining member before it starts dqlite

[jrtorsella]

This PR addresses an issue where microcluster called into go-dqlite's Join() logic before a cluster member was trusted by all other members. Go-dqlite attempted to reach the other cluster members, determined because they were unreachable that they were offline, and would attempt to promote the joining member on that basis, before finishing the join.

The changes in this PR are:
1) Before responding to the joiner with the join token in
clusterPost(), the leader calls propagateTrustStoreEntry() (which
is factored out of the relevant logic where this work is usually
done so that it can be called in this case). This sends the
joiner's trust store entry to all the members in the cluster on a
best-effort basis, warning if it fails or takes longer than 10
seconds and continuing.
2) The 'pending' member record in the database for the new node is
not modified and its lifecycle does not change, but it is
handled differently by the code that builds the trust store entry
lists that are sent by the leader to each member each heartbeat.
Previously, the leader ignored a pending entry, and built the
trust store list based only on non-pending entries. With this
change, pending entries also form part of the heartbeat's list of
trusted members.
3) The joiner no longer attempts to send its own entry to every
member. Previously this was a catch-22 situation and ineffective.

fixes #825 .
https://warthogs.atlassian.net/browse/LXD-5252?atlOrigin=eyJpIjoiNzM4NmU0MTc1MDM3NDcxMGIyYTE3YTA0YjllMTk0YzkiLCJwIjoiaiJ9

[Copilot]

🟢 Approval recommended

The join ordering is corrected with focused coverage; only minor error-message wording remains.

1 open finding

• Use established "Failed to <verb>" error wording · New

What changed in this PR

Ensures joining members are trusted cluster-wide before dqlite starts.

Changes:

• Propagates joiner trust entries from the leader.
• Includes pending members in heartbeat trust-store updates.
• Removes ineffective post-join trust propagation and adds tests.

File	Description
internal/​rest/​resources/​truststore.go	Extracts trust propagation logic.
internal/​rest/​resources/​truststore_test.go	Tests propagation scenarios.
internal/​rest/​resources/​heartbeat.go	Retains pending members in trust stores.
internal/​rest/​resources/​cluster.go	Propagates trust before returning join data.
internal/​daemon/​daemon.go	Removes late joiner-side propagation.

🧠 Review effort: Balanced

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