Closes #189.

Adds fluent helpers to telegram/message for picking and sending stickers from the various sticker sources, matching the API sketched in the issue.

API

sender.Self().Sticker(message.FavedStickers()).First(ctx)
sender.Self().Sticker(message.RecentStickers()).ByEmoji(ctx, "😎")
sender.Self().Sticker(message.StickerSetName("AnimatedEmojies")).ByIndex(ctx, 0)

Sources (StickerSource)

Each is backed by the corresponding messages.* RPC:

• FavedStickers() — messages.getFavedStickers
• RecentStickers() / AttachedRecentStickers() — messages.getRecentStickers
• StickerSet(set) / StickerSetName(shortName) — messages.getStickerSet

Selection (*StickerSetBuilder, via Builder.Sticker(source))

• ByIndex(ctx, i, caption...)
• ByEmoji(ctx, emoji, caption...) — matches DocumentAttributeSticker.Alt
• First(ctx, caption...)
• Stickers(ctx) — exposes the resolved documents

Sending reuses the existing Document(...) media path (a sticker is just an InputMediaDocument); *NotModified/empty results surface clear errors.

Notes

• ISSUES.md lists message: sticker helpers #189 as already implemented via telegram/query/cached, but that only covers the fetch/cache side — the send-side helpers the issue asked for didn't exist. This fills that gap.

Tests

Offline via tgmock: index/emoji selection, out-of-range, emoji-not-found, set-by-name, and the NotModified error path. Plus a usage example. go test ./telegram/message/ -race and go build ./... pass.

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