Add support for subChannels being stored in entities

[benbrown]

This PR adds support for the ProductInfo entity, and adds some helper functions around the use of the channelId field.

Resolves #533

New:

• channelId will now be a combination of primary channel (legacy channelId) + the subchannel id from the entity.
• channelIdChannel will return on the primary channel
• channelIdSubChannel will return the subchannel id

activity.channelId = 'foo:bar' works
activity.channelIdChannel = 'foo' works
activity.channelIdSubChannel = 'bar' works

How this works:
Under the hood, we have getter/setter functions for channelId and the other sub-fields.
The "legacy" value of channelId from the wire is stored in _channelId.
The entity is stored as transmitted.
Deserialization works as expected with no change.

During serialization, we move the value from _channelId to channelId. This happens in both toJsonString and in the noramalizeOutgoingActivity method.

GOTCHAS:
If a user does JSON.stringify(activity) they will end up with _channelId instead of channelId

[benbrown]

TO DO:

• Add jsdoc comments
• Address copilot comments
• Find places where we are already doing activity.channelId == and update to channelIdChannel

[Copilot]

Pull Request Overview

Adds support for representing subchannels via a ProductInfo entity and introduces getters/setters to combine primary channel and subchannel into a composite channelId while normalizing serialization to keep only the primary channel in wire payloads.

• Introduces ProductInfo entity and helper for adding it to activities.
• Replaces direct channelId storage with internal _channelId plus computed accessors (channelId, channelIdChannel, channelIdSubChannel).
• Normalizes outgoing activities to replace _channelId with channelId and updates serialization logic.

Reviewed Changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 7 comments.

File	Description
packages/agents-hosting/src/activityWireCompat.ts	Normalizes outgoing payload by moving _channelId to channelId.
packages/agents-activity/src/activity.ts	Adds internal _channelId plus composite channelId accessors and updated serialization logic.
packages/agents-activity/src/entity/productInfo.ts	Defines ProductInfo entity and helper for attaching it to an Activity.
packages/agents-activity/test/activity/activity.subChannels.test.ts	Adds tests covering composite channelId behavior and serialization.

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[tracyboehrer]

Passing Activity.channelId to the various UserTokenClient operation could cause an issue. Unclear if ABS will handle the full channelId. DotNet will use just the "channel" portion for those calls.