Expose issueTokens helper and plumb runtime site URL

[smaccoun]

Sorry to be that guy, but this is a mostly AI generated PR. I did review it though and it seems reasonable to me. At a minimum would like to get the conversation going

Motivation

We rely on Convex Auth for passwordless flows where a user redeems a magic link and should land in the app already authenticated (think clinician emailing a patient an assessment link). In hosted/E2E setups the internal.auth.store mutation runs in a worker that never sees runtime process.env mutations, so token minting fails with “Missing environment variable CONVEX_SITE_URL.” That breaks the magic-link UX and forces clients to call internal APIs to work around it. Pushing the site URL through config and exposing a supported issueTokens helper keeps these flows working across environments without relying on internals.

Summary

• fix auth:store workers skipping token minting when runtime processes can't see CONVEX_SITE_URL by threading site/custom auth url data through call stack
• normalize site URL handling so JWT issuer, OAuth redirects, and cookie names all use config overrides (or CUSTOM_AUTH_SITE_URL fallback) instead of direct process.env reads
• provide a public issueTokens(actionCtx, {userId, sessionId?}) helper so apps avoid internal.auth.store and still get tokens with the right issuer metadata; add the convex test hook covering it

Testing

• not run

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