Add GHE support for gist resolution with GH_HOST

[harshithvh]

Summary

• GH_HOST environment variable
• URL-based host detection
• GitHub CLI-compatible API patterns using /api/v3/ endpoints
• Dual pattern support: subdomain (gist.enterprise.com) and path (enterprise.com/gist/)

Thanks for this (#15109 (comment)) @jorgehermo9 🙏

closes #15109

Test Plan

• URL-based vs environment-based detection scenarios
• integration tests for all Enterprise patterns

[jorgehermo9]

As I commented in #15109 (comment) I think we shouldn't use the GH_HOST env var unless we add something like uv run --gist <gist_id>, because with what we have right now, we already know the host we should use, as the input is the gist's web url and not only the id. Allowing the GH_HOST would be pointless in my opinion (at least right now)

[harshithvh]

As I commented in #15109 (comment) I think we shouldn't use the GH_HOST env var unless we add something like uv run --gist <gist_id>, because with what we have right now, we already know the host we should use, as the input is the gist's web url and not only the id. Allowing the GH_HOST would be pointless in my opinion (at least right now)

Hm, I don't see the harm/downside in keeping GH_HOST support, when we add uv run --gist <gist_id> enterprise users will need GH_HOST. Removing it now = breaking change later.