feat: add filters.IS_BOT to filter messages by sender bot status - #5380

Summary

Closes #5315 — adds filters.IS_BOT to filter messages by whether the sender is a bot.

Changes

• filters.py: Added _IsBot MessageFilter class and IS_BOT constant
• filters.py: Added IS_BOT to __all__ exports
• test_filters.py: Added test_filters_is_bot test covering: bot user match, negation, and from_user=None edge case
• changes/unreleased/: Added changelog entry

API

from telegram.ext import filters

# In a handler:
@router.message(filters.IS_BOT)
def handle_bot_message(update, context): ...

# Negation works as expected:
@router.message(~filters.IS_BOT)
def handle_human_message(update, context): ...

Note: Channel posts and anonymous-admin messages have no from_user and therefore do not match; use filters.SenderChat for those.

Acceptance criteria

• filters.IS_BOT exists and filters by User.is_bot
• Negation via ~filters.IS_BOT works
• Edge case from_user=None handled correctly
• Tests added and passing
• Changelog entry added