Fix two resource-management bugs in StartDeployWatcher / BuildDefaultHierarchySource:
(a) Replace the discarded `_ = StartAsync(...)` with an explicit task variable that
surfaces any synchronous InvalidOperationException (called-twice guard) rather than
silently swallowing it.
(b) Change both StartDeployWatcher and BuildDefaultHierarchySource to use ??= on
_ownedRepositoryClient so the first client created (by whichever path runs first)
is reused by the second path, preventing a second GalaxyRepositoryClient from being
created and the first from leaking past the driver's lifetime.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
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