Allow in-place Chat history editing

Nobody has claimed this yet.

`lmstudio-js` supports in-place Chat history editing via:

* offering both mutable and immutable chat history and chat message instances via an internal mutability flag (using the [`MaybeMutable` base class](https://github.com/lmstudio-ai/lmstudio-js/blob/05df1a2284304e20ed931a125f5b4d06a21483df/packages/lms-client/src/Chat.ts#L369))
* the [`.replaceText()` method](https://github.com/lmstudio-ai/lmstudio-js/blob/05df1a2284304e20ed931a125f5b4d06a21483df/packages/lms-client/src/Chat.ts#L709) on saved messages (there are also `.appendFile()` and `appendText()` methods on messages)
* the [`.replaceSystemPrompt()` method](https://github.com/lmstudio-ai/lmstudio-js/blob/05df1a2284304e20ed931a125f5b4d06a21483df/packages/lms-client/src/Chat.ts#L369) to remove all previously added system prompt messages and insert the given prompt at the start of the chat history
* the [`.filterInPlace()` method](https://github.com/lmstudio-ai/lmstudio-js/blob/05df1a2284304e20ed931a125f5b4d06a21483df/packages/lms-client/src/Chat.ts#L375) to process all currently saved messages with the given filtering callback
* the [`.pop()` method](https://github.com/lmstudio-ai/lmstudio-js/blob/05df1a2284304e20ed931a125f5b4d06a21483df/packages/lms-client/src/Chat.ts#L221)
* supporting direct iteration over and indexed access to the messages saved in the chat history

The Python SDK should offer similar capabilities (although the exact spelling may differ).

Use the linked lmstudio-js Chat.ts implementation as the behavioral reference, then locate the corresponding chat history and message APIs in the Python SDK. Define how mutable and immutable instances, text replacement, system-prompt replacement, filtering, popping, iteration, and indexed access should work, and verify the completed behavior with the repository's existing tests.

Written by the indexing model from the issue text.