sessions: recover empty Agent Host session list after a failed initial refresh (#319216)

* sessions: recover empty Agent Host session list after a failed initial refresh

The Agents-app local session list could stay empty on launch and only
populate after the user started a new session. The provider's session
cache is one-shot: `_refreshSessions()` ran once, but its blanket catch
could not tell "listSessions() threw" (e.g. the agent throws
AHP_AUTH_REQUIRED before its token is effective server-side, or a
transient offline error) from "listSessions() returned []" (a genuinely
empty, valid result). Both collapsed to an empty cache marked
initialized, and nothing retried until an unrelated AHP event
(SessionTurnComplete / sessionAdded) forced a re- which is whyfetch
starting a new session "unstuck" the list.

`_refreshSessions` now owns `_cacheInitialized` and sets it only on a
successful list. On a thrown failure it logs and arms a capped
exponential-backoff retry (1s -> 30s, reset on success), so a transient
startup failure self-heals on its own. An empty successful list is still
treated as a valid result that marks the cache initialized and arms no
retry. Listing sessions still never pops a sign-in dialog;
AHP_AUTH_REQUIRED is retried silently in the background.

The local provider's auth autorun and the remote provider's
setConnection no longer pre-set `_cacheInitialized`; clearConnection
cancels any pending retry.

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

* fix compile error: declare retry backoff statics before instance field

The static SESSION_REFRESH_RETRY_MIN_MS was referenced by the
_sessionRefreshRetryDelay instance field initializer but declared after
it, causing a 'used before its initialization' compile error under the
full tsc build.

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

* address review: guard _ensureSessionCache against redundant refreshes

With _cacheInitialized now staying false until a successful listSessions,
every synchronous getSessions()/getSessionByResource() during the failure
window would call _ensureSessionCache() and launch a fresh listSessions(),
bypassing the backoff and repeatedly hitting the agent/auth path. Add an
in-flight flag and skip _ensureSessionCache when a refresh is already
running or a backoff retry is already scheduled, so recovery only happens
via the backoff timer.

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

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