🐛 Add cash_balance property to FakePortfolio (#63)

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<details open="true"><summary>Generated summary (powered by <a href="https://app.graphite.dev">Graphite</a>)</summary>

> ## TL;DR
> This pull request introduces a new property `cash_balance` to the `Portfolio` class in `conftest.py` under the tests directory.
> 
> ## What changed
> A new property `cash_balance` was added to the `Portfolio` class. This property returns a float value of 0. This change is reflected in the `conftest.py` file.
> 
> ```python
> @Property
> def cash_balance(self) -> float:
>     return 0
> ```
> 
> ## How to test
> To test this change, you can create an instance of the `Portfolio` class and access the `cash_balance` property. The expected return value should be 0. 
> 
> ```python
> portfolio = Portfolio()
> print(portfolio.cash_balance)  # should print 0
> ```
> 
> ## Why make this change
> This change was made to provide a way to access the cash balance of a portfolio. This can be useful in scenarios where we need to check if the portfolio has enough cash balance to make a transaction.
</details>

tests/conftest.py

@@ -16,6 +16,10 @@ def __init__(self) -> None: ...
     def total_value(self) -> float:
         return 100000
 
+    @property
+    def cash_balance(self) -> float:
+        return 0
+
     @property
     def positions(self) -> list[Position]:
         return [