Making ProblemPrompt.get_prompt return str not StrEnum

[jamesbraza]

ProblemPrompt.get_prompt is typed to return a str, and in a few cases it returns XMLAnswerPrompts variants.

XMLAnswerPrompts.REASONING_ANSWER is a StrEnum, which for all normal usage works fine. Actually the existing TestChatArguments and the newly added test_problem_prompt work both before and after this PR, and also note mypy passes too.

However, I felt it weird that ProblemPrompt.get_prompt would return a StrEnum in some cases, so this PR updates that.

[Copilot]

Pull Request Overview

This PR ensures that ProblemPrompt.get_prompt consistently returns a plain str by extracting the .value from any StrEnum cases and adds tests to validate this behavior.

• Returned .value for THINK_ANSWER and ANSWER cases instead of the enum members
• Introduced test_problem_prompt to assert string outputs for each ProblemPrompt variant

Reviewed Changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
tests/test_model_prompts.py	Added test_problem_prompt to verify get_prompt returns a str and contains expected keywords
src/ether0/model_prompts.py	Updated get_prompt to use .value on XMLAnswerPrompts variants