This fixes HerokuProvider base URL normalization when the configured Heroku inference URL already includes /v1.

What Problem This Solves

HerokuProvider stripped trailing slashes and always appended /v1:

base_url = base_url.rstrip('/') + '/v1'

That works for the default https://us.inference.heroku.com, but it turns an already-normalized OpenAI-compatible endpoint into a malformed double-version path:

https://us.inference.heroku.com/v1  -> https://us.inference.heroku.com/v1/v1/
https://us.inference.heroku.com/v1/ -> https://us.inference.heroku.com/v1/v1/

Users may naturally pass a /v1 endpoint through HerokuProvider(base_url=...) or HEROKU_INFERENCE_URL, since Heroku Managed Inference exposes OpenAI-compatible /v1/... API paths.

Change

• Normalize the Heroku base URL once.
• Append /v1 only when the configured base URL does not already end with /v1.
• Add a regression test covering base URLs with and without /v1, with and without a trailing slash.

Evidence

Before this change, local reproduction showed:

https://us.inference.heroku.com/v1  -> https://us.inference.heroku.com/v1/v1/
https://us.inference.heroku.com/v1/ -> https://us.inference.heroku.com/v1/v1/

After this change, all supported inputs normalize to:

https://us.inference.heroku.com/v1/

Reference: Heroku Managed Inference documents OpenAI-compatible /v1 API endpoints.

Validation

• uv run pytest tests/providers/test_heroku.py::test_heroku_provider_normalizes_base_url
• uv run ruff check pydantic_ai_slim/pydantic_ai/providers/heroku.py tests/providers/test_heroku.py
• uv run ruff format --check pydantic_ai_slim/pydantic_ai/providers/heroku.py tests/providers/test_heroku.py
• git diff --check

Checklist

• Any AI generated code has been reviewed line-by-line by the human PR author, who stands by it.
• No breaking changes in accordance with the version policy.
• PR title is fit for the release changelog.