fix: fetch payment sessions from database for persistence

The test provider was using an in-memory Map to store payment sessions,
which caused "Payment session not found" errors in several scenarios:

1. Next.js hot reload clearing the memory
2. Different execution contexts (API routes vs Payload admin)
3. Server restarts losing all sessions

This fix updates all three test provider endpoints (UI, process, status)
to fetch payment data from the database when not found in memory:

- Tries in-memory session first (fast path)
- Falls back to database query by providerId
- Creates and caches session from database payment
- Handles both string and object collection configurations

This makes the built-in test UI work reliably out of the box, without
requiring users to implement custom session management.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

package.json

@@ -1,6 +1,6 @@
 {
   "name": "@xtr-dev/payload-billing",
-  "version": "0.1.20",
+  "version": "0.1.21",
   "description": "PayloadCMS plugin for billing and payment provider integrations with tracking and local testing",
   "license": "MIT",
   "type": "module",