Fix IMAP connection exhaustion under burst of searches

IMAP read/mutation operations opened a fresh connection per call and closed
it via Dispose() without a graceful LOGOUT. Lingering connections plus repeated
AUTHENTICATE attempts hit Gmail's per-account connection limit/throttle, stalling
subsequent AUTHENTICATE calls so rapid searches hung for the full timeout.

Reuse a single authenticated IMAP connection per account behind a per-account
gate, with NOOP health-check, reconnect-and-retry on a broken reused connection,
and graceful DisconnectAsync(true) on eviction/shutdown via IAsyncDisposable.

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: rockfordlhotka

Directory.Build.props

@@ -1,5 +1,5 @@
 <Project>
   <PropertyGroup>
-    <VersionPrefix>1.4.0</VersionPrefix>
+    <VersionPrefix>1.4.1</VersionPrefix>
   </PropertyGroup>
 </Project>

changelogs/2026-06-09-imap-connection-reuse.md

@@ -0,0 +1,45 @@
+# 2026-06-09: IMAP Connection Reuse & Graceful Disconnect
+
+## Summary
+
+Fixes a bug where rapid, repeated IMAP operations (e.g. a burst of
+`search_emails` calls) would hang for the full request timeout against Gmail.
+
+Every IMAP operation previously opened a brand-new connection, ran a fresh
+`AUTHENTICATE`, and then closed the connection by `Dispose()` **without** a
+graceful `LOGOUT`. Abruptly-dropped connections linger server-side and count
+against Gmail's per-account simultaneous-connection limit, and Gmail throttles
+repeated `AUTHENTICATE` attempts. Under a burst of searches the limit/throttle
+kicked in and stalled the next `AUTHENTICATE`, so each subsequent call hung. SMTP
+(`send_email`) was unaffected because it already disconnected gracefully and uses
+a different protocol with different limits.
+
+## Fix
+
+`ImapProviderService` now keeps a single authenticated IMAP connection **per
+account** and reuses it across calls:
+
+- A per-account `SemaphoreSlim` gate serializes access (an `ImapClient` runs one
+  command at a time).
+- Each call health-checks the cached connection with a lightweight `NOOP` and
+  transparently reconnects if it is dead, with a single reconnect-and-retry if a
+  reused connection breaks mid-operation.
+- Connections are disconnected **gracefully** (`DisconnectAsync(true)` →
+  `LOGOUT`) when evicted or on service shutdown, so the server releases the slot
+  immediately.
+- `ImapProviderService` implements `IAsyncDisposable`/`IDisposable` so the DI
+  container tears down pooled connections cleanly.
+
+This eliminates the repeated `AUTHENTICATE` round-trips that Gmail throttles and
+prevents connection-slot exhaustion.
+
+## Scope
+
+Read and mutating IMAP operations now share the pooled connection:
+`get_emails`, `search_emails`, `get_email_details`, `get_email_attachment`,
+`delete_email`, `mark_email_as_read`, `move_email`, and the APPEND-to-Sent step
+of `send_email`.
+
+## Backward compatibility
+
+No API or behavior changes for callers. All existing tests pass.