fix: allow PUT tenant to clear database_pool_url and database_pool_mode

[oniani1]

The PUT /tenants/:id handler used truthy guards on databasePoolUrl and databasePoolMode, so a request with null would skip the assignment and the merge upsert left the existing column unchanged. The schema declares both fields nullable: true, and the PATCH handler at lines 396-401 already does the three-way check.

This switches both checks to !== undefined and treats explicit null as a clear, mirroring PATCH. tenantDBInterface was widened to allow string | null on the two columns so the assignment typechecks.

admin-tenants.test.ts adds a case that PUTs a tenant with both pool fields populated, verifies the encrypted/string values land in the DB, then PUTs again with null for both and asserts the columns are now NULL. Verified locally against multitenant_db running in Docker; the case fails on master and passes with the fix.

Closes #1068

[coveralls]

Coverage Report for CI Build 25131948326

Coverage increased (+0.3%) to 72.266%

Details

• Coverage increased (+0.3%) from the base build.
• Patch coverage: 3 of 3 lines across 1 file are fully covered (100%).
• No coverage regressions found.

Uncovered Changes

No uncovered changes found.

Coverage Regressions

No coverage regressions found.

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Coverage Stats

Relevant Lines:	9823
Covered Lines:	7497
Line Coverage:	76.32%
Relevant Branches:	5530
Covered Branches:	3598
Branch Coverage:	65.06%
Branches in Coverage %:	Yes
Coverage Strength:	392.99 hits per line

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