Add a throttle-aware Microsoft Graph HTTP layer (org.libremail.mail.graph) and route
the live me/sendMail path through it, composing with #360's AccountThrottleGate:
- GraphHttpClient: the single Graph HTTP transport seam; preserves the send path's
may-have-sent distinction (GraphTransportException) and parses Retry-After.
- GraphThrottle: caps Graph concurrency at 4, honors a 429/503 Retry-After via the
shared per-account backoff gate (retry after the honored wait, bounded), clears it
on a 2xx. Because the gate is shared+account-keyed, a Graph 429 also cools that
account's IMAP background work down.
- GraphBatch: multiplexes ops via $batch (<=20/call), collapsing N calls to
ceil(N/20); feeds per-op 429s back into the gate.
- GraphUploadSession: createUploadSession + chunked Content-Range PUTs for content
over Graph's ~4 MB one-shot ceiling (320 KiB-multiple chunks).
- GraphSender.send now honors a Graph 429 once (Retry-After) before falling back to
SMTP, instead of failing over on the first throttle.
Outlook mail is read over IMAP and the Graph token is Mail.Send-scoped, so $batch
reads and draft-based chunked attachment upload have no live call site yet (they need
the Mail.ReadWrite scope, a re-consent-forcing change kept out of this perf ticket);
both ship as fully-tested capabilities of the Graph layer.
Unit tests (MockK the HTTP client, coroutines-test virtual time, no real sleeps) cover
429+Retry-After backoff, $batch call-count reduction, and chunked upload; an
instrumented test exercises the toolkit under the Android runtime. PII-free AppLog
(accountLogRef) throughout; SPDX on every file.
Closes#364
JMR-dev