fix: rebalance pool in place

[ferhatelmas]

Fixing Two issues in tenant pool lifecycle:

1. Cluster rebalance destroyed and recreated every cached pool, causing GC spikes.
2. When a tenant's DB URL or credentials changed, the cached pool kept its stale connections — updateAgeOnGet prevented TTL eviction, so requests kept
   failing against a broken pool.

Changes

• Cluster scale → in-place. TenantPool.rebalance mutates tarnPool.max and nudges _tryAcquireOrCreate(). No destroy, no GC churn.
• onTenantConfigChange picks the right teardown:
  • databaseUrl / databasePoolUrl changed → hard destroy. Next request rebuilds against the new endpoint.
  • maxConnections changed → graceful recycle. New pool swapped into the cache synchronously; old pool drains in the background.
  • Pool-mode flip → hard destroy (existing).
• New PoolManager.recycle(tenantId, settings) — synchronous cache swap, background drain via TenantPool.destroy(). Uses manuallyDestroyedPools
  to keep the cache dispose hook from racing the explicit destroy.

[Copilot]

Pull request overview

This PR changes tenant DB pool rebalancing to update existing Knex/Tarn pools in place (instead of draining/destroying and recreating), to avoid cache evictions and reduce GC churn during cluster autoscaling or tenant max-connection updates.

Changes:

• Update TenantPool.rebalance() to modify the underlying Tarn pool’s max value in place.
• Trigger Tarn’s acquire/create loop after resizing (when available) to apply the new max.
• Update the unit test to assert the pool/Knex instance is preserved across rebalance and that destroy() is not called.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
src/internal/database/pool.ts	Switches rebalance behavior from “replace/drain pool” to “resize Tarn pool max in place”.
src/internal/database/pool.test.ts	Adjusts expectations to validate in-place resizing and no pool destruction during rebalance.

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[coveralls]

Coverage Report for CI Build 26514061357

Coverage increased (+0.1%) to 75.265%

Details

• Coverage increased (+0.1%) from the base build.
• Patch coverage: 2 uncovered changes across 1 file (24 of 26 lines covered, 92.31%).
• No coverage regressions found.

Uncovered Changes

File	Changed	Covered	%
src/internal/database/tenant.ts	8	6	75.0%
Total (2 files)	26	24	92.31%

Coverage Regressions

No coverage regressions found.

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Coverage Stats

Relevant Lines:	10654
Covered Lines:	8449
Line Coverage:	79.3%
Relevant Branches:	6241
Covered Branches:	4267
Branch Coverage:	68.37%
Branches in Coverage %:	Yes
Coverage Strength:	365.15 hits per line

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💛 - Coveralls