fix(pg-cache): allow cache to reopen after close for process survival

[pyramation]

fix(pg-cache): allow cache to reopen after close for process survival

Summary

Fixes a server crash: Error: Cannot add to cache after it has been closed (key: postgres).

PgPoolCacheManager.close() permanently set closed = true with no reset path. When SIGTERM fires but the process doesn't exit (e.g., during provisioning or restart under load), all subsequent getPgPool() calls throw because set() is guarded by the closed flag. The module-level closePromise was also never reset, preventing future close() cycles from running.

Changes (2 lines of logic in postgres/pg-cache/src/lru.ts):

• Reset this.closed = false after waitForDisposals() completes in PgPoolCacheManager.close(), so the cache can accept new entries post-shutdown
• Wrap module-level close() body in try/finally to reset closePromise.promise = null, matching the existing pattern in graphile-cache.ts

Review & Testing Checklist for Human

• Concurrent shutdown race: Verify that a set() call arriving during an active close() (between this.closed = true and this.closed = false) still correctly throws, and only succeeds after disposal completes
• Repeated SIGTERM: Confirm that receiving a second SIGTERM after the cache reopens triggers a proper new close cycle (the closePromise reset + closed = false should allow this)
• Manual test: Kill and restart the server process under load during provisioning to confirm the crash no longer occurs

Notes

• The fix is intentionally minimal — two assignments and a try/finally wrapper
• The graphile-cache package already uses this finally reset pattern (line 283 of graphile-cache.ts); this PR aligns pg-cache with that convention
• Trailing newline added to end of file (formatting only)

Requested by: @pyramation
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