Move context str generation into configurable function

[mskarlin]

Users may want custom control over how contexts are generated and munged before inserting into the final answer. This was previously locked into the aquery code, but this exposes it as a configurable function.

We may not be the best, but by god, we'll be the most configurable.

[Copilot]

Pull Request Overview

This PR refactors the context string generation logic to make it configurable through a function parameter. Previously, the context generation was hardcoded within the aquery method, but now users can provide custom functions to control how contexts are processed and formatted before being inserted into the final answer.

Key changes:

• Extracted context string generation logic into a configurable context_str_fn parameter
• Created a ContextStrFn protocol to define the interface for custom context functions
• Added a default implementation that preserves the original behavior

Reviewed Changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 1 comment.

File	Description
src/paperqa/settings.py	Adds ContextStrFn protocol and context_str_fn field to AnswerSettings with proper type configuration
src/paperqa/docs.py	Refactors aquery method to use configurable context function and extracts original logic to default_context_str_fn
tests/test_paperqa.py	Adds test coverage for custom context string function functionality

[jamesbraza]

Nice work overall, thanks for going through the PR comments