Add unit tests for chat history in FirebaseAI

[ryanwilson]

This commit introduces two new unit tests to FirebaseAI/Tests/Unit/ChatTests.swift:

1. testChatHistory(): This test verifies the chat history functionality when starting with a pre-existing history. It ensures that new messages (both user and model) are correctly appended to the history.
2. testChatHistoryWithEmptyInitialHistory(): This test covers the scenario where the chat starts with an empty history. It validates that the first user message and the subsequent model response correctly initialize and populate the history.

Both tests utilize MockURLProtocol to simulate network responses, ensuring reliable and isolated testing of the chat history logic.

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