Harden Outlook mailbox pool polling and health checks

[AaronL725]

Summary

Strengthen the Outlook mailbox-pool implementation across protocol correctness, retry behavior, lifecycle management, and diagnostics.

Mail polling

• Replace IMAP message-count/sequence tracking with UIDVALIDITY + UID cursors.
• Parse IMAP special-use folders and modified UTF-7 names, quote mailbox names safely, and cache the prepared folder set/connection during the OTP window.
• Monitor Microsoft Graph Inbox and JunkEmail with immutable message IDs plus received-time frontiers instead of totalItemCount.
• Keep Graph listing lightweight and fetch message bodies only for newly crossed frontier items.
• Keep uninspected Graph messages retryable after transient detail-fetch failures and guard against cross-folder replay.

Reliability and security

• Tighten verification-code extraction to explicit OTP context or trusted xAI/Grok sender domains.
• Reuse preflight access tokens and IMAP connections instead of immediately refreshing again.
• Add bounded Microsoft HTTP concurrency, Retry-After handling, exponential backoff, jitter, and transient retry coverage.
• Make Outlook opaque mailbox handles one-shot and release credential-bearing mailbox state immediately after polling finishes or is cancelled.
• Preserve independent pre-send channel baselines: a channel without a safe baseline remains disabled for that registration attempt.

Diagnostics and UI

• Add safe Outlook mailbox-pool health checks to both GUI and WebUI.
• Health checks validate IMAP/Graph access without starting an xAI registration and return only safe account/channel metadata.
• Keep the Web health endpoint local-only, non-cacheable, and serialized against registration/maintenance work.

Validation

• Expanded Outlook regression coverage for UID cursor behavior, UIDVALIDITY changes, Graph JunkEmail, cross-folder moves, transient detail-fetch failures, OTP false positives, token reuse, Retry-After, one-shot handles, cancellation cleanup, health APIs, and secret hygiene.
• Full test suite passes on Python 3.9 and 3.13 (290 tests), along with compile checks, JavaScript syntax checks, diff hygiene, and Outlook-specific invariant checks.

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