GO-7561 concurrent AddLog of the same log returns ErrLogExists instead of WriteConflict

[requilence]

Problem

Every sync node that stores a space calls AddLog and then Watch when it loads the space, usually at the same time as the other replicas. On stage (2026-09-30 to 10-05) nodes logged failed to add consensus record: (WriteConflict) … WiredTigerIdIndex::_insert: index: _id_, each followed within 1 ms by received consensus error: log not found for the same space.

1. Since v0.6.0 AddLog runs in a transaction. When several nodes create the same log at once, the losers fail with WriteConflict while the winner is still uncommitted. That error went out joined with ErrUnexpected, and drpc cannot read a code from errors.Join, so it reached the client with code 0 and the raw mongo text.
2. The loser then watches the log before the winner commits and gets ErrLogNotFound. The client keeps its watcher registered and only logs the error, so the node got no ACL pushes for that space until it unloaded it or the watch stream reconnected.

Fix

• db: tx runs a transaction again on TransientTransactionError (up to 10 attempts, 10 ms linear backoff), so a loser waits for the winner's commit and gets ErrLogExists; its watch then finds the log. Transactions return mongo errors as they are, so tx can see their labels; AddLog, AddRecord and DeleteLog then replace any uncoded error with ErrUnexpected (code 500), logging the original. AddLog and AddRecord no longer clear the payloads of their input, which a retried transaction needs again.
• stream: a missing log is looked up once more. A watch can join a load that another watch started before the log was committed and share its ErrLogNotFound; ocache drops a failed load, so the second lookup reads the db again.
• stream, defensive: a failed watch drops the log id before the error is sent. Before, the id stayed in the stream's watched ids although the stream was never attached, so a repeated WatchIds without UnwatchIds was skipped. consensusclient does not send that (UnwatchIds already removed the id), so this changes nothing for current clients; the error still ends the watch.

Remaining gap: if AddLog still fails (every retry exhausted), the node's watch ends with ErrLogNotFound and nodeSpace.AddConsensusError only logs it, as before. The node keeps its ACL and gets new records through headsync, without the push. A re-watch on the node would close this; not part of this PR.

An earlier version kept a watch on a missing log waiting for it. That was dropped: clients treat ErrLogNotFound as final, and the coordinator's acl watcher panicked on records after it.

Tests

• stream: TestStream_WatchMissingLog checks the event order a client sees (second lookup, error ends the watch, re-watch, other errors); all four cases fail on main.
• db: concurrent AddLog of the same log (fails on main with the stage WriteConflict), tx retry with injected labeled errors (retry, wrapped label, exhaustion, no retry of other errors), AddLog keeps its input, error conversion keeps code 500.

The db tests need MongoDB as a replica set; they were run against a single-node replica set.

Deploy

Requires anyproto/any-sync#805 in the coordinator, any-sync-filenode and any-sync-filenode2 first. consensusclient restores its watches after a reconnect outside its mutex; an UnWatch + Watch of the same log in that window, followed by the restore, sends the id twice on one stream. Before, the second request was skipped; now both get ErrLogNotFound, which an unpatched acl watcher turns into a double close. The window is narrow, and the same panic is reachable on main without this PR. Order: coordinator and filenodes → consensusnode → nodes (anyproto/any-sync-node#251, optional).

Known limits: retries are bounded by attempts, not time (a server-selection failure inside a transaction is also labeled transient); an unknown commit result is handled as before.

Part of GO-7561.

[github-actions]

New Coverage	69.7% of statements
Patch Coverage	97.4% of changed statements (38/39)

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