Allow the JSTOR integration to load JSTOR's open access (free) articles without needing a site code

**The Problem**
JSTOR (and Artstor) have "Open Access" articles which are free to access, even by users without JSTOR accounts. See this open access article: https://www.jstor.org/stable/2974201. Contrast with this non-free article: https://www.jstor.org/stable/3325946

Currently our JSTOR integration seems to require a valid JSTOR site code to work with any JSTOR text (investigation in Slack [here](https://hypothes-is.slack.com/archives/C2C2U40LW/p1747253993813549)).

1) We're unable to turn on the JSTOR integration for Hypothesis partners who don't have a JSTOR site code, even though they could potentially access JSTOR's Open Access content.

2) We're unable to access JSTOR's content with an invalid site code.

**The solution**
Working with JSTOR, we could either:
- Use a single site code with any Hypothesis partners who don't currently use JSTOR. This site code would allow access to Open Access texts only.
- Use a per-school site code that only has access to Open Access texts, so JSTOR can track usage per school
- If JSTOR wants to allow this, but doesn't want to give us a site code, investigate changes to our current JSTOR integration so that the JSTOR option appears in the assignment creation menu without needing a site code, and doesn't check against JSTOR's API when there's not a site code entered.

Slack thread: https://hypothes-is.slack.com/archives/C02T75RKBTK/p1747339536675579