CBL-8202: Fix expiration logic on SQLITE_BUSY

[borrrden]

Also fix the expiration torture test to not exit early on the same condition

[github-actions]

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Please confirm this PR is one of the following:

• A response to a customer ask
• A change per our security policy
• A non-functional change (i.e. changes needed for building an older version)
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[cbl-bot]

Code Coverage Results:

Type	Percentage
branches	64.71
functions	77.41
instantiations	70.98
lines	76.04
regions	72.3

[Copilot]

Pull request overview

This PR addresses a failure mode in LiteCore’s background document expiration where a transient SQLITE_BUSY during the purge could prevent future expirations from being scheduled, and updates the associated “expiration torture” test to tolerate transient busy failures under artificial contention.

Changes:

• Wrap Housekeeper::_doExpiration() in a try/catch to detect LiteCore/Busy and reschedule a near-term retry instead of letting the actor swallow the exception and skipping future scheduling.
• Add a short retry delay constant for busy-induced expiration retries.
• Update the C API torture test to retry c4coll_setDocExpiration on transient kC4ErrorBusy instead of aborting early.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 4 comments.

File	Description
LiteCore/Database/Housekeeper.cc	Catch-and-retry expiration purge on Busy to keep expiration scheduling alive under transient lock contention.
C/tests/c4DatabaseTest.cc	Make the expiration torture test retry setDocExpiration when it transiently returns Busy.

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