Support for answer iterations in the gen_answer flow

[mskarlin]

If we are answering after a prior answer already exists in the session, we currently don't expose that to the gen_answer tool / aquery. To support continuations, where a user follows up to a query, exposing the prior answer if very helpful (because the agent can't pass it directly), so this implements that feature.

Pull Request Overview

This PR adds support for answer iterations in the gen_answer flow so that a prior answer can be employed when a user follows up on a query. Key changes include:

• Updating tests to verify that both initial queries and queries with a prior answer are handled correctly.
• Adding new iteration prompts in prompts.py to instruct the model on how to build on a previous answer.
• Updating settings and prompt configuration to include iteration-related defaults and validations.

Reviewed Changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated no comments.

File	Description
tests/test_paperqa.py	Added tests to validate query behavior when a prior answer is provided.
paperqa/prompts.py	Introduced new prompts for answer iterations.
paperqa/docs.py	Updated query logic to insert iteration context into the prompt.
paperqa/settings.py	Added iteration-related settings and a field validator for the new prompt.