refactor(channels): return uppercase Twilio channel type from get_channel_name

[xinghaohuang91]

Summary

get_channel_name() now returns the uppercase ChannelType Literal ("SMS", "VOICE", "RCS", "WHATSAPP", "CHAT") that matches Twilio's channel enum, instead of a lowercase str. This lets the channel string flow through webhook filtering, API payloads, and session storage in a single consistent form — no more .upper()/.lower() juggling and no type-checker friction at ParticipantAddress(channel=...).

What changed:

• Removed get_channel_type_upper() and every .upper()/.lower() conversion on the channel name.
• Centralized the channel-type Literal, previously duplicated across 5 model fields, into one shared ChannelType alias in models/conversation.py.
• Typed ConversationSession.channel and get_channel_name() as ChannelType.
• Simplified the dashboard example (session.channel is now always populated, so the lowercase fallback helper was dead code).

⚠️ Breaking change: ConversationSession.channel and get_channel_name() now return uppercase (e.g. "SMS" not "sms"). Consumers comparing session.channel == "voice" must switch to "VOICE".

Type of Change

• Breaking change
• Refactoring

Checklist

• Tests added/updated
• Documentation updated
• Tested E2E

SDK Parity

This is the Python SDK. This change affects shared channel-type behavior — the TypeScript SDK should be reviewed for parity.

• Change is Python-specific (no TypeScript update needed)
• TypeScript SDK PR created:

[Copilot]

Pull request overview

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This PR standardizes channel identifiers across the Python SDK by having get_channel_name() and ConversationSession.channel use Twilio’s uppercase channel enum (via a shared ChannelType Literal), removing ad-hoc casing conversions and a now-redundant helper.

Changes:

• Introduced a shared ChannelType alias and updated models to use it for channel fields.
• Refactored channels to have get_channel_name() return uppercase ChannelType and removed get_channel_type_upper().
• Updated tests and the dashboard example to expect/use uppercase channel strings consistently.

Reviewed changes

Copilot reviewed 29 out of 29 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
src/tac/models/conversation.py	Adds shared ChannelType Literal and reuses it in multiple model fields.
src/tac/models/session.py	Types ConversationSession.channel as ChannelType with uppercase examples.
src/tac/models/memory.py	Replaces duplicated channel Literal[...] with shared ChannelType.
src/tac/models/tac.py	Replaces duplicated channel Literal[...] with shared ChannelType.
src/tac/channels/base.py	Makes get_channel_name() return ChannelType and removes .upper() usage for filtering/logging.
src/tac/channels/messaging.py	Removes get_channel_type_upper() and uses get_channel_name() for initiation.
src/tac/channels/{sms,rcs,whatsapp,chat}.py	Updates get_channel_name() implementations to return uppercase ChannelType.
src/tac/channels/voice/channel.py	Updates get_channel_name() to return "VOICE" as ChannelType.
src/tac/tools/handoff.py	Updates voice channel comparison to "VOICE".
getting_started/examples/features/dashboard/dashboard.py	Removes dead channel fallback and directly returns session.channel.
tests/*	Updates expectations/fixtures from lowercase to uppercase channel strings and removes get_channel_type_upper() assertions.

Comments suppressed due to low confidence (1)

src/tac/channels/messaging.py:1

• The class docstring still states get_channel_name() returns a lowercase channel name, but the refactor changes it to return uppercase Twilio channel types via ChannelType (e.g., "SMS", "RCS", "WHATSAPP", "CHAT"). Update this bullet (and any similar doc text in this file) to match the new contract so developers don’t implement/compare against lowercase values.

"""MessagingChannel base class for messaging channels (SMS, RCS, WhatsApp, Chat)."""

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