fix: misidentified the connected user as the removed group member

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[linear-code]

PLAT-38628

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PR Summary

Fixes LINE member-removal (C_MR/A_MR) system-message handling, which misidentified msg.From (the remover) as the removed member. When the connected user removed someone, the bridge removed the connected user instead, causing the whole group chat to disappear from Beeper (PLAT-38628). The removed member is now taken from LOC_ARGS, and the leave is attributed to the removed member with the remover as the change sender. Later commits bump the Go toolchain to 1.26 and migrate a deprecated mautrix-go API so the updated linter passes.

• Adds memberRemovalMIDs to derive the remover (msg.From) and the removed member (last distinct user MID in LOC_ARGS).
• makeSystemMessageEvent emits the leave for the removed member with the remover as change sender, and evicts the correct cache entry; isHandledSystemMessage only accepts removal records carrying a valid removed user MID distinct from the remover.
• Adds tests for attribution, cache eviction, and validation of malformed records.
• Bumps Go to 1.26 (go.mod, deploy workflow, Dockerfile, AGENTS.md).
• Migrates the deprecated ChatMemberList.Members slice to MemberMap via a new chatMemberMapFromList helper (create-group, DM/group resync, chat-info paths); behavior-preserving since both key by sender.
• Suppresses staticcheck SA1019 on the required rsa.EncryptPKCS1v15 call used by LINE's login protocol.

Issues

Review closed.

CI Checks

All CI checks passed on 4a259e4.