fix(engine): preserve replay sequence after send failure

[willwashburn]

Prevents an agent's higher-sequence durable delivery from being sent after its lower-sequence replay fails. Other agents on the same node continue replaying normally.\n\nAdds conformance coverage for partial provider failure and records the patch-level behavior in the root and engine changelogs.\n\nValidated with the focused delivery suite (53 tests), engine build, and engine lint.

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[miyaontherelay]

Assessment: still needed, not superseded

Checked whether relay#1579 ("join Relaycast messages into completed replay",
merged 2026-08-20) supersedes this. It doesn't — different subsystem
entirely. #1579 is about SessionClient.replaySession() joining a
Relayhistory turns journal with the Relaycast conversation for
human-readable session replay. This PR is about deliverPendingToNode's
durable-delivery redelivery — preventing a higher-sequence frame from
reaching a provider before a lower-sequence one that failed to send,
which would otherwise put a permanent gap in one agent's ordered message
stream. No code or data overlap between the two.

Current state: draft, but MERGEABLE/CLEAN against main, and every
check is green (Container amd64/arm64, Lint/Build/Test, Rust SDK). It's
just been sitting untouched since 2026-08-18 — 4 days with no reviewer
and no one hitting "ready for review."

Recommend: un-draft and merge, not close. A correctness fix for message
ordering that's green and conflict-free shouldn't go stale waiting on
someone to notice it.