Support checkpointing and reverting file changes in AI sessions

[begelundmuller]

This PR adds two new APIs:

• Adds an API SetConversationCheckpoint, which tracks a checkpoint in an AI session. This enables remembering at what point in a conversation the user last explicitly marked changes as accepted.
• Adds an API RevertConversationWrites, which reverts any file writes/deletes made in a (subset) of an AI session. This enables reverting changes either for a specific write_file tool call, or for all write_file tool calls made since the latest checkpoint.

Internals:

• Checkpoints are cheap to create – they just change a pointer to a message ID.
• When a file is created/updated/deleted, the previous contents are stored in the write_file tool call's state, enabling easy reverts.
• The full previous contents gets redacted before any LLM call so it doesn't pollute the context and use unnecessary tokens.

Remaining tasks (can be done in follow ups):

• To prevent unbounded data growth, remove file contents snapshots when a checkpoint is created or after 24 hours.

Here's the full request parameters and response payload:

// Request message for RuntimeService.SetConversationCheckpoint
message SetConversationCheckpointRequest {
  // Instance the conversation belongs to.
  string instance_id = 1 [(validate.rules).string = {pattern: "^[_\\-a-zA-Z0-9]+$"}];
  // Conversation ID to checkpoint.
  string conversation_id = 2;
  // Message ID of the latest message *before* the checkpoint.
  string message_id = 3;
}

// Response message for RuntimeService.SetConversationCheckpoint
message SetConversationCheckpointResponse {}

// Request message for RuntimeService.RevertConversationWrites
message RevertConversationWritesRequest {
  // Instance the conversation belongs to.
  string instance_id = 1 [(validate.rules).string = {pattern: "^[_\\-a-zA-Z0-9]+$"}];
  // Conversation ID to revert writes for.
  string conversation_id = 2;
  // Start message ID to revert from (inclusive). If empty, it reverts from after the conversation's checkpoint.
  string from_message_id = 3;
  // End message ID to revert to (inclusive). If empty, it reverts to the latest message.
  string to_message_id = 4;
}

// Response message for RuntimeService.RevertConversationWrites
message RevertConversationWritesResponse {
  repeated string restored_paths = 1;
}

Closes https://linear.app/rilldata/issue/PLAT-358/checkpointingreverting-ai-writes

Checklist:

• Covered by tests
• Ran it and it works as intended
• Reviewed the diff before requesting a review
• Checked for unhandled edge cases
• Linked the issues it closes
• Checked if the docs need to be updated. If so, create a separate Linear DOCS issue
• Intend to cherry-pick into the release branch
• I'm proud of this work!