fix: LTSM aborts caused encryption notices and a failed image send

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[linear-code]

PLAT-38468

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PR Summary

Handles fatal LTSM (WASM) aborts as a recognizable error so they stop triggering spurious encryption/"re-login" notices and a failed image send, and hardens the media decrypt and request-signing paths along the way.

• Adds ltsm.ErrAbort sentinel (_abort panics with it); isFatalLTSMError in the E2EE manager skips the V1 fallback on a V2 abort, and the inbound/outbound message paths short-circuit key refetch, retry, markMissingE2EEKey, and plaintext fallback when an abort is detected.
• Consolidates the per-type audio/file/image/video decrypt logic into one decryptDownloadedMedia helper: tries the body keyMaterial then ENC_KM, passes genuinely-plain media through, and fails closed with ErrIgnoringRemoteEvent (retryable, no error notice) consistent with the existing mediaDownloadFailure pattern.
• Reworks request signing to derive and cache an HMAC per access token in a 4-entry LRU, adds WASM destructor support (Destroy/SecureKeyDestroy/HmacDestroy), and frees evicted objects plus the transient objects in Sign/DebugExportDerivedSigningKey to fix WASM memory leaks.
• Adds unit/integration tests for abort classification, media decrypt behavior, HMAC reuse/eviction, and destructor dispatch.

Issues

Review closed.

CI Checks

All CI checks passed on 2a712df.