feat: stable sessions

[ascorbic]

Changes

This PR unflags the session API. Aside from removing the flag, this changes some of the details of how session configuration is validated, because of them no longer being opt-in. Previously, if a user had enabled the flag and not configured a driver we would throw when validating the config We can no longer do this because we don't know if the sessions will be used. Instead, we use a getter for the session object in context and the Astro global. Previously this would just be undefined. Now it also logs a warning if accessed, explaining why it is undefined. As well as doing this when there is no driver, it will catch attempts to access it in prerendered pages. This would previously fail with a headers error when it tried to set a cookie. It now fails earlier, with a more helpful error message.

Testing

The configuration tests have mostly been removed, as they no longer apply. Other tests have been updated to remove flags.

Docs

This will have a lot of docs changes, to come later.

[github-actions]

This PR is blocked because it contains a minor changeset. A reviewer will merge this at the next release if approved.

[codspeed-hq]

CodSpeed Performance Report

Merging #13527 will not alter performance

_{Comparing stable-session (47fb9cf) with main (7d251c1)}

Summary

✅ 6 untouched benchmarks

[changeset-bot]

🦋 Changeset detected

Latest commit: 47fb9cf

The changes in this PR will be included in the next version bump.

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[sarah11918]

Just a few tiny suggestions here, changeset and config reference! Looking pretty sweet!

[sarah11918]

Approving for docs! 🥳

(Noting I did review the configuration-reference JSDoc and it looks fine to the eye here. We'll see for sure when it's pulled into docs proper.)

Merge day