[chain-pr 5/12] Rewrite session storage strategies as reactive setSessionState API

[johnHackworth]

This PR is part of a chain split from #0 v7 by chain-pr.

Changes

Replace persistSession/retrieveSession/expireSession with a single setSessionState(fn) + sessionObservable on each strategy. Adds CookieAccess abstraction with CookieStore API and document.cookie fallback. Migrates session key to _dd_s_v2 with legacy migration. Removes oldCookiesMigration and sessionStoreOperations.

Refactoring rationale

Fundamentally restructures the session storage layer from a synchronous persist/retrieve/expire trio into a single reactive setSessionState method backed by an observable. This unifies cookie, localStorage, and memory strategies under one contract, replaces the lock-based concurrency mechanism with Web Locks / promise chains, and bumps the storage key to _dd_s_v2 with legacy migration. It is the foundational change required before the new session manager can be built on top.

---

Draft — pending author review. Merge in order unless marked parallel.

[github-actions]

Thank you for your submission, we really appreciate it. Like many open-source projects, we ask that you sign our Contributor License Agreement before we can accept your contribution. You can sign the CLA by just posting a Pull Request Comment same as the below format.

---

I have read the CLA Document and I hereby sign the CLA

---

chain-pr[bot] seems not to be a GitHub user. You need a GitHub account to be able to sign the CLA. If you have already a GitHub account, please add the email address used for this commit to your account.
_{You can retrigger this bot by commenting recheck in this Pull Request. Posted by the CLA Assistant Lite bot.}