fix(docroom): guard lastsync writes against DO 128 KiB value cap

[kptdobe]

Summary

Durable Object storage rejects values > 131072 bytes with a RangeError. Both lastsync writes in src/shareddoc.js — persistence.update after a successful da-admin PUT, and persistence.bindState after the da-admin restore fetch — used to call storage.put('lastsync', …) unconditionally and surface the platform-deterministic throw as [docroom] Failed to write lastsync … console.error noise (sustained 2 events / 24h on the da-collab error stream, two distinct payload sizes per day).

Introduce a small safePutLastsync(storage, value, docName, context) helper that:

• Skips the put entirely when value.length > MAX_STORAGE_VALUE_SIZE, logging the skip at console.log level so it stays out of the error channel.
• Routes any other exception class through logError(...), so the existing demotion path for isExpectedPlatformEvent (DO live migration / worker upgrade events) still applies.

Both call sites delegate to the helper, so any future lastsync write inherits the guard for free.

Why

The 128 KiB cap is platform-deterministic in the same way that the live-migration error is — there is no point landing it in console.error. The pre-existing try/catch comment promised "non-fatal: worst case the restore falls back to da-admin"; in practice that fallback was already taking effect (the marker was skipped after the throw), but the throw was producing log noise on the error channel for every oversize save. Skipping is safe because the absence of the lastsync key is already the documented trigger for the da-admin restore on DO restart.

The pattern matches the demotions shipped in #135 / #155 / #157 for the live-migration / upgrade events.

Test plan

• Failing-test-first: new mocha cases in test/shareddoc.test.js mock a storage stub that throws the production RangeError shape (Values cannot be larger than 131072 bytes. A value of size <N> was provided.). Both persistence.update and persistence.bindState integration cases for oversize content were verified to fail against an inline-revert of the call-site updates before the guard was applied.
• After the fix: npx mocha --reporter spec --import=./test/stubs/register.mjs --exit test/*.test.js — 165 passing, 0 regressions.
• Direct helper tests cover oversize, in-cap, and no-storage cases.
• npm run lint — clean.
• Post-deploy: confirm m.contains('Values cannot be larger than') errors at $d.ScriptName == 'da-collab', l.Level == 'error' return to 0 / 24h × 3 consecutive days.

Scope notes

• No change in lastsync semantics for ≤ 128 KiB content — the existing "preserve pending Yjs changes across DO evictions" guarantee is preserved (verified by the existing bindState writes lastsync … test, which still passes unchanged).
• Not addressing whether oversize documents should chunk the marker — current behavior (rely on da-admin restore fallback) is unchanged.

[adobe-bot]

🎉 This PR is included in version 1.7.3 🎉

The release is available on:

• v1.7.3
• GitHub release

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