[yeroo]

Closes #1899

What and why

A Microsoft Graph delta item may carry only some properties ({id, isRead} for a read change, a bare
{id} for an item created and changed between two rounds). Message::from_json defaults every absent
field to empty, and upsert_message overwrote every column, so new mail landed with no subject, sender
or date, and existing rows could be blanked.

• MessageFields (graph/model.rs) records which properties a JSON item carried (a JSON null
  counts as carried). DeltaItem::Upsert is now Upsert(Message, MessageFields). This changes a public
  enum; nothing outside mailcore matches on it.
• Store::merge_message writes only the carried columns: one static INSERT … ON CONFLICT DO UPDATE SET col = CASE WHEN ?present …. It handles every delta upsert. upsert_message now delegates to it
  with every field present, so there is a single statement. The meeting-request and draft flags are
  written when their key is present or the item is complete, which keeps today's behaviour for complete
  items.
• Repair (Engine::repair_headerless) runs after each folder's delta. It handles up to 50 rows
  per folder per pass that have received_at = '' and are not docxy's own (excluded: local: ids,
  is_draft = 1, and local_id IS NOT NULL, i.e. drafts reconciled and then sent).
  • Each row is fetched with GET /me/messages/{id}?$select=… and merged.
  • A 404 deletes the row.
  • A 4xx or parse error skips that id and the repair continues.
  • Auth, throttle or transport errors abort the pass, as the delta call does.
  • This completes new header-less items in the same pass and repairs rows that older builds blanked.
• Events. One SyncEvent::Error per folder names how many header-less rows are being fetched. A
  partial update merged into a complete row is silent. MessagesUpdated is sent once the delta is
  stored, before the repair, and again if the repair changed a row.

Tests

16 new tests across model, client, store and engine. The engine tests:

• partial_delta_item_keeps_headers_and_fetches_new_item, the issue's acceptance case: M1 keeps its
  headers with the new isRead and is not refetched; M3 is fetched and stored complete; one notice.
• headerless_row_is_repaired_on_next_pass
• repair_deletes_row_graph_no_longer_has
• repair_continues_past_a_failing_id
• throttled_repair_still_reports_the_folders_delta

The store tests:

• upsert_writes_every_column, which also fails if the schema gains a column the test does not cover
• the draft and meeting-flag parity tests
• headerless_ids_skip_reconciled_sent_draft

The implementer checked that the engine tests fail when the old upsert path is put back.

All of these pass: cargo fmt --check, cargo clippy -p mailcore -p lookxy --all-targets -- -D warnings,
cargo test -p mailcore (300 passed) and cargo test -p lookxy (333 passed). lookxy needed no change.

Review record

• Plan: two rounds with the implementer. v2 added the single write path, the local_id exclusion,
  and skipping past a failing id; the implementer's draft-flag addendum was accepted.
• revmux round 1: 4 Minor findings, 0 Major, all fixed in 251f676.
  • duplicated upsert SQL
  • the repair abort suppressing MessagesUpdated
  • the notice counting merged read changes
  • the notice being an Error; kept as the issue asks, and now rare and documented
• review stopped: round 1 had no Major; all findings fixed

🤖 Generated with Claude Code

[codecov-commenter]

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Codecov Report

❌ Patch coverage is 99.50434% with 4 lines in your changes missing coverage. Please review.

Files with missing lines	Patch %	Lines
mailcore/src/graph/model.rs	97.97%	2 Missing ⚠️
mailcore/src/store/mod.rs	99.70%	1 Missing ⚠️
mailcore/src/sync/engine.rs	99.68%	1 Missing ⚠️

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[yeroo]

Merged automatically (auto-merge is on for this checkout): review stopped: round 1 had no Major; all findings fixed; merge rounds: none; whole suite green on 251f676 (633 tests: cargo fmt --check, clippy -D warnings and cargo test for mailcore and lookxy; CI 14/14 passed); merge-check ok.

No follow-ups: all four Minor findings from revmux round 1 were fixed in this PR, nothing deferred or disputed.