Competing approach with existing open PR #9500, both addressing issue #3634 (Telegram channel_post updates silently dropped). This PR takes a different approach using effective_message and also adds chat type normalization.

Thanks for flagging. I looked at #9500 and agree they overlap on #3634, but this PR is intentionally a bit broader:

• fix: handle Telegram channel-post commands #9500 is scoped to channel-post slash commands and adds a channel-post command handler/helper.
• This PR fixes the shared handler assumption by reading update.effective_message, so the same path covers normal messages and channel posts for text, commands, locations, and media handlers that PTB dispatches to those filters.
• It also fixes channel-post identity when from_user is absent, so channel allowlisting/session attribution can use the numeric channel ID instead of ending up with a missing user identity.
• It includes chat type normalization because the channel fallback depends on reliably recognizing ChatType.CHANNEL across real PTB enum values and test/plain-string values.

I also checked current python-telegram-bot filter behavior locally: filters.TEXT and filters.COMMAND both match a synthetic Update.channel_post, so the main drop appears to be handlers reading update.message instead of effective_message, not missing registration for the current PTB version. If maintainers prefer the more explicit filters.UpdateType.CHANNEL_POSTS & ... registration style from #9500 for readability/defense-in-depth, I can fold that in too, but I avoided adding duplicate handlers since the existing filters already match channel posts.

Merged via PR #28531. Authorship preserved (rebase-merge). #28531