Add multiple iterations test for TokenRateLimitPolicy

[emmaaroche]

Description

This PR adds a test to verify that the TokenRateLimitPolicy correctly enforces its limits and properly resets after multiple iterations.

This PR also restructures the TRLP tests to separate shared and story-specific fixtures:

trlp/
  ├── user_story/
  │   ├── conftest.py
  │   └── test_trlp.py
  ├── conftest.py
  └── test_multiple_trlp_iterations.py

Related issue

• Closes Add multiple iterations test for TokenRateLimitPolicy #748

[averevki]

>           assert (
                response.status_code == 429
            ), f"Iteration {i+1}: Expected 429 after {total_tokens} tokens, but got {response.status_code}"
E           AssertionError: Iteration 9: Expected 429 after 13 tokens, but got 200
E           assert 200 == 429
E            +  where 200 = <testsuite.httpx.Result object at 0x7fc2cc2798b0>.status_code

Failed for me one time randomly. Time to tokens relation might be a bit off for this test. But lets merge it and see how it runs on nightlies over this weekend. I'd prefer that and a smaller change later than to block this PR for a few more days. Good work! 😺