Make LocalAddressBook's group-membership helpers suspend

[rfc2822]

Another step of #2828 (taking #2784 into account).

Purpose

Continues making at.bitfire.davdroid.resource.local (the Local* classes between SyncManager and the content providers) a proper suspending API.

Short description

• Made LocalAddressBook's group-membership lookup helpers (used by applyPendingMemberships()) suspend, each dispatching its content-provider call onto Dispatchers.IO.
• Moved the raw group-membership query out of core into a new synctools method, wrapping RemoteException as LocalStorageException like its siblings.
• Fixed two other Local* methods that were already suspend but never actually dispatched their content-provider access onto Dispatchers.IO.
• Documented findByName()'s exception behavior.

Checklist

• The PR has a proper title, description and label.
• I have self-reviewed the PR.
• I have added documentation to complex functions and functions that can be used by other modules.
• I have added reasonable tests or consciously decided to not add tests.

[Copilot]

Pull request overview

This PR continues the effort to make the core local-storage layer (at.bitfire.davdroid.resource.local) a properly suspending, non-blocking API by moving contacts-provider queries into Dispatchers.IO and centralizing provider-access helpers in synctools.

Changes:

• Added a new AndroidAddressBook.getContactIdsByGroupMembership() helper in synctools, wrapping RemoteException as LocalStorageException.
• Converted LocalAddressBook group-membership lookup helpers (and related lookups) to suspend functions and dispatched their provider access onto Dispatchers.IO.
• Fixed LocalJtx* methods that were suspend but still performed provider access without Dispatchers.IO, and documented findByName()’s exception behavior at the interface level.

Reviewed changes

Copilot reviewed 5 out of 5 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
synctools/src/main/kotlin/at/bitfire/synctools/storage/contacts/AndroidAddressBook.kt	Adds a reusable group-membership query helper and wraps provider RemoteException into LocalStorageException.
core/src/main/kotlin/at/bitfire/davdroid/resource/local/LocalAddressBook.kt	Makes group-membership helper calls suspend + IO-dispatched; switches to the new synctools helper.
core/src/main/kotlin/at/bitfire/davdroid/resource/local/LocalJtxObject.kt	Ensures JTX update writes run on Dispatchers.IO.
core/src/main/kotlin/at/bitfire/davdroid/resource/local/LocalJtxCollection.kt	Ensures JTX find-by-name provider access runs on Dispatchers.IO.
core/src/main/kotlin/at/bitfire/davdroid/resource/local/LocalCollection.kt	Documents LocalStorageException behavior for findByName().

Suppressed comments (1)

core/src/main/kotlin/at/bitfire/davdroid/resource/local/LocalAddressBook.kt:323

• todo: findGroupById() can now throw LocalStorageException, but the @Throws annotation only lists FileNotFoundException.

ab.getGroupOrNull() wraps provider failures as LocalStorageException, which will propagate through this API. If you intend to keep @Throws for Java callers, include LocalStorageException here as well.

    @Throws(FileNotFoundException::class)

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