fix: validate SOGS attachment sizes

[Ap4sh]

Contributor checklist:

• My commits are in nice logical chunks with good commit messages
• My changes are rebased on the latest dev branch
• The relevant checks pass
• My changes are ready to be shipped to users

Description

Refs #1953

This hardens SOGS attachment downloads by requiring message attachments to include an expected size before they are accepted as successful downloads

Community avatar downloads still opt into unknown-size payloads explicitly, since that path does not get size metadata

Tests:

• corepack pnpm exec tsc
• corepack pnpm exec eslint --cache .
• corepack pnpm exec cross-env IS_UNIT_TEST=1 mocha
• corepack pnpm dedupe --check
• corepack pnpm build:protobuf
• corepack pnpm build:sass
• corepack pnpm build:svg
• corepack pnpm worker:utils
• corepack pnpm worker:libsession
• corepack pnpm worker:image_processor

[MikeScanlan5]

Code looks clear and fixes the intended attachment size issue.