Refactor getModels to centralize model definitions and avoid duplicate registrations

[AZ0228]

Motivation

• Prevent repeated model re-registration on the same DB connection and centralize model metadata to reduce duplication and errors.
• Improve performance and robustness by lazily creating models and validating that req.db is present.

Description

• Introduced a frozen MODEL_DEFINITIONS map that contains modelName, schema, and collection for each model instead of recreating the full models object per call.
• Added registeredConnections as a WeakSet to track DB connections that have had their models registered and an ensureAllModelsRegistered helper to register models once per connection.
• Added getOrCreateModel to return an existing model if present or create it with req.db.model(...) and updated getModels to validate req.db and return only requested models using the centralized definitions.
• Preserved public API by exporting getModels and kept original collection names and schema usages.

Testing

• No automated tests were executed as part of this change.

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Codex Task