Sync v2: Remove direct integration with set_if_absent

[CDRussell]

Task/Issue URL: https://app.asana.com/1/137249556945/project/1203822806345703/task/1216036247181808?focus=true
Tech Design URL (if applicable):

Description

Drop the direct POST /sync/keys/.../set-if-absent call from the 3party credential creation flow which previously existed as its own distinct step. Removing this also means we can remove some now-redundant dev tooling for it.

Steps to test this PR

• Exact same as Sync exchange v2: Fix password deriation mismatch #9014

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Note

Medium Risk
Changes the sync v2 crypto path for first-time ai_chats keys and 3party credential setup; mistakes could break scoped login or key consistency, though behavior is localized to the 3party credential flow behind the v2 feature flag.

Overview
3party credential creation no longer calls POST /sync/keys/.../set-if-absent or uses ProtectedKeyManager. When the account has no ddg-wrapped ai_chats key, the client mints an RSA keypair via mintDdgWrappedProtectedKey and includes both ddg and 3party ProtectedKeyEntry rows (same kid) in the POST /access-credentials/3party keys[] payload, alongside re-wrapped existing ddg keys.

The setProtectedKeyIfAbsent Retrofit/API surface, SyncAccountRepository.createProtectedKey, ProtectedKeyManager, and related tests/dev UI (“Create Protected Key”) are removed. ThirdPartyCredentialManager adds bounded retries on concurrent_modification 409s for that credential POST and broadens 409 handling messaging before the adopt-existing-credential path.

^{Reviewed by Cursor Bugbot for commit b0288d5. Configure here.}

[CDRussell]

• Replace thread.sleep in sync exchange v2 with coroutine-based delay #8986
• Sync v2: Remove direct integration with set_if_absent #9012 👈 (View in Graphite)
• Sync exchange v2: Fix password deriation mismatch #9014
• develop

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[CDRussell]

will switch to coroutine / delay in higher branch

[CrisBarreiro]

Looks good but retry on "concurrent_modification" and “stop after max retries” branches could use some testing

[CDRussell]

Merge activity

• Jun 26, 4:45 PM UTC: A user started a stack merge that includes this pull request via Graphite.
• Jun 26, 4:47 PM UTC: Graphite rebased this pull request as part of a merge.
• Jun 26, 4:59 PM UTC: @CDRussell merged this pull request with Graphite.