fix(chat): expand grouped media exports

[expary]

Problem

tdl chat export currently writes only the message that directly matches the filter. For grouped media albums, Telegram stores each media item as a separate message sharing the same grouped ID. If only one item in the album contains the matched text, the export JSON includes only that item, so a later tdl dl -f export.json misses the rest of the album.

Changes

• add an export message expander that runs the filter on the iterated message
• when the matched message belongs to grouped media, resolve the whole group with tutil.GetGroupedMessages
• export every downloadable message in the matched group
• deduplicate message IDs so later iterator hits from the same group are not written twice
• preserve existing --all, --with-content, and --raw export behavior for each emitted message

Tests

• go test ./app/chat -count=50
• go test ./app/chat ./app/dl ./pkg/tmessage
• go test $(go list ./... | grep -v '^github.com/iyear/tdl/test$') ./core/... ./extension/...

Closes #1207