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## 🧭 Paging Cursor Enhancements: Nullable Keys and Null Ordering Support

### Summary

This PR introduces enhancements to the paging logic to correctly handle sorting and filtering when cursor keys are nullable. The main change is that the `WHERE` condition used in pagination must now adapt based on **three factors**:

1. Whether the **cursor key** is nullable.
2. Whether the **actual cursor value** is `null` or not.
3. Whether the **database** sorts `null` values **first** or **last**.

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### 🔍 Why This Is Needed

When performing cursor-based pagination on fields that can be `null`, it's essential to construct the `WHERE` clause correctly. Incorrect assumptions about `null` ordering or the presence of `null` values can result in missing or duplicated records across pages.

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### 🔧 Implementation Details

To support this behavior, the following changes were made:

- **Cursor Key Metadata**
  - Cursor keys now include a new property (`IsNullable`) to indicate whether they are nullable.

- **Null Ordering in Cursor**
  - Cursors now carry a `NullsFirst` flag to indicate how `null` values are ordered.
  - For subsequent pages, this flag is **inherited** from the previous cursor.
  - For the **first(last) page**, the flag is **calculated** based on whether the **first(last) item** contains a `null` value:
    - If a `null` is found → we can determine the null ordering: nulls first(nulls last).
    - If not → we assume the opposite ordering.
      - If this assumption is incorrect, it does **not affect correctness**, because it implies there are **no `null` values** in the dataset, making null ordering irrelevant.

> The concept of handling nullable types in relative cursor-based pagination is inspired by [[this StackOverflow discussion](https://stackoverflow.com/questions/68971695/cursor-pagination-prev-next-with-null-values)].

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### ✅ Benefits

- Correct pagination over nullable fields.
- Eliminates edge cases where `null` records might be skipped or duplicated.
- Supports both `nulls first` and `nulls last` configurations in the database.

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### ⚠️ Side Effects / Limitations

If our assumption about null ordering is incorrect, **secondary ordering keys** may appear to be sorted in reverse (opposite to the database's actual null handling behavior), particularly when paginating **backward from the last page**.

This only affects the **visual or logical order** of `null` values relative to each other and **does not break pagination**—all items will still be included in the correct pages.