fix(sysdb): serialize collection deletion with writes

[mikemikimike]

Summary

delete_collection used a deferred SQLite transaction while create_collection already acquired the write lock immediately. Under concurrent create/delete operations, the delete could read stale state and both requests could report success.

This change applies the existing begin_immediate helper to collection deletion, serializing the read-modify-write sequence with other collection mutations.

Validation

• cargo fmt --check
• git diff --check
• The focused Rust test command was attempted, but dependency resolution was blocked by an unavailable git submodule (googleapis/googleapis) required by gcloud-gax.

[claude]

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[github-actions]

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Testing, Bugs, Errors, Logs, Documentation

• Can you think of any use case in which the code does not behave as intended? Have they been tested?
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• If appropriate, are there adequate property based tests?
• If appropriate, are there adequate unit tests?
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Quality

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