feat: new ContextFilterPlugin

[copybara-service]

feat: new ContextFilterPlugin

Introduces a new ContextFilterPlugin in Java, which is designed to reduce the size of the LLM context by filtering out older turns.

Here's a breakdown:

• ContextFilterPlugin.java: This new plugin extends BasePlugin and implements logic in its beforeModelCallback method. It trims the LlmRequest contents to retain only a specified number of the most recent "model" turns and their preceding "user" turns. A key aspect is the adjustSplitIndexToAvoidOrphanedFunctionResponses method, which ensures that any FunctionResponse included in the filtered context has its corresponding FunctionCall also present, preventing invalid inputs to the LLM.
• ContextFilterPluginTest.java: This file contains unit tests for ContextFilterPlugin, verifying its behavior for both scenarios where context filtering is not needed and where it successfully reduces the LlmRequest contents based on the configured number of invocations to keep.