fix: handle error 99 (member count mismatch)

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[linear-code]

PLAT-38749

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PR Summary

Fixes LINE error 99 ("member count mismatch") during E2EE group key registration by registering against LINE's authoritative server snapshot instead of assembling the member list locally.

• registerGroupKey now fetches getLastE2EEPublicKeys(chatMid) (which includes the caller), wraps the new group key for exactly those members, and registers; on a "member count mismatch" it refreshes the snapshot and retries once (bounded).
• Adds IsE2EEGroupMemberMismatch to detect TalkException code 99 / reason "member count mismatch".
• Changes GetLastE2EEPublicKeys to take a bare chatMid (single positional RPC arg) and parse data via json.RawMessage.
• Removes the now-unneeded local member-resolution machinery (groupKeyMemberMIDs, resolveGroupMemberPublicKeys, getChatMemberMIDs, joinedGroupMemberMIDs, getGroupMemberMIDsViaMatrix) and their tests, plus the GetLastE2EEPublicKeysRequest struct.

Issues

All clear! No issues remaining. 🎉

2 issues already resolved

• Group message send fails hard instead of falling back to plaintext when a member lacks a usable E2EE key. The rewritten registerGroupKey returns its validation errors ("incomplete E2EE group member public key", "no registration members returned", "members do not include caller") without the line.ErrNoUsableE2EEGroupKey wrapper, so lineGroupE2EEFetchFailureError/IsNoUsableE2EEGroupKey on the send path no longer classify them and return the error to the user. Wrap them with line.ErrNoUsableE2EEGroupKey to restore the plaintext fallback. (fixed by commit c28fd7d)
• The comment above the groupMemberCache write in CreateGroup still says the list is cached "so auto-registration can fall back to it," but this PR removed those auto-registration fallbacks. The cache is now only consumed by name generation in sync.go; update the comment to avoid misleading future readers. (fixed by commit c28fd7d)

CI Checks

All CI checks passed on c28fd7d.