fix(auth): do not await PowerSync disconnect during session clear

[bentucker]

Proposed Changes

• Stop awaiting the PowerSync disconnect in the keep-data session reset
  (clearSessionOnly). The await closes a three-party circular wait when a
  refresh token is rejected: the refresh future runs the session teardown,
  the teardown awaits PowerSync's disconnect, the disconnect waits for the
  in-flight sync fetch to abort, and that fetch is awaiting the same
  single-flight refresh future through AuthHttpClient's pre-emptive refresh.
  The refresh future then never completes and every later authenticated
  request queues behind it forever (15 second timeouts on every endpoint,
  surviving app restarts). See the issue for the full production diagnosis.
• The disconnect is now fire-and-forget on this path. The DB is deliberately
  kept, so there is no wipe to race with: once the cleared state is
  published, the wedged sync fetch resumes without a credential, gets a 401
  and PowerSync backs off, letting the deferred disconnect finish. The wipe
  path (explicit logout) keeps its wipe-before-publish ordering.
• Make the disconnect hook injectable (visibleForTesting field) so the
  regression test can install a never-completing disconnect;
  PowerSyncDatabase is a base class and cannot be faked directly.
• Add a regression test: a rejected refresh must complete and publish the
  logged-out state even when the PowerSync disconnect hangs forever.

Related Issue(s)

Closes #1312

Checklist

• Tests pass locally (flutter test, 1235 tests)
• flutter analyze reports no issues on the changed files
• New behaviour is covered by a regression test