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Related issue

Closes #6167

Summary

This change narrows the lock scope for packaged preindexed source opens.

Previously, opening a packaged preindexed source always acquired the named cross-process lock before doing the read-only open work. Under concurrent winget usage, that serialized source reads behind unrelated activity and caused large stalls before commands like show or search could do meaningful work.

This PR changes the packaged-context open path to:

• attempt the packaged source open optimistically first
• reuse a helper for extension lookup, content integrity verification, and immutable SQLite open
• fall back to the existing cross-process lock only if the optimistic open fails
• preserve cancellation behavior
• add timing/logging around the open path to make the mode and cost visible

The change is limited to PreIndexedPackageSourceFactory.cpp.

Testing

Manual validation against the issue repro:

• compared behavior against concurrent packaged source reads
• verified the fallback path still retries under the existing lock when optimistic open fails

Static validation:

• no diagnostics reported for the touched file

Microsoft Reviewers: Open in CodeFlow