fix(astro): persist session delete without prior get/set

[enjoyandlove]

Closes #16563

Summary

Calling session.delete(key) as the first mutation in a request (no prior get, set, has, keys, etc.) did not write back to session storage. The session stayed dirty in memory, but [PERSIST_SYMBOL]() skipped the save path because #data was still undefined, so the backing store kept the old value and the next request could still read the “deleted” key.

Root cause

• set() initializes the in-memory map with this.#data ??= new Map(); delete() only did this.#data?.delete(key), so #data could remain undefined.
• Persistence gated saves on if (this.#dirty && this.#data), so delete-only flows never called setItem and #toDelete was never applied to storage.

Fix

• Initialize the map in delete() the same way as in set(): this.#data ??= new Map() before removing the key, so the persist path runs and #ensureData() can merge, apply deletions, and serialize to the driver.

Testing

• Added a unit test that pre-seeds backing storage, calls only delete() then persist, and asserts a new AstroSession with the same storage/session id no longer returns the deleted key.
• Adjusted an existing persistence test so the follow-up session’s storage get returns parsed JSON (matching real unstorage behavior) after setItem writes a devalue JSON string.

Docs

No documentation change required; behavior now matches what the session API already promises.

Contributor checklist (see CONTRIBUTING.md)

• pnpm exec changeset for the astro package (user-facing bugfix)
• Tests: pnpm -C packages/astro exec astro-scripts test "test/units/sessions/astro-session.test.ts"

[changeset-bot]

🦋 Changeset detected

Latest commit: 60ed956

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[enjoyandlove]

Hi, @MatthewLymer

I’ve just submitted this PR. When you have a moment, could you please review it?

Thank you for your time

[codspeed-hq]

Merging this PR will not alter performance

✅ 18 untouched benchmarks

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_{Comparing enjoyandlove:fix/session-delete-persist-without-prior-load (60ed956) with main (9ac96b4)}

[enjoyandlove]

Hi, @matthewp
All checks have passed. When you have a moment,
could you please review this PR?
Thank you for your time.