fix: DM conversation naming issue for one-way single-participant conversations

[ChristopherJHart]

Problem

Previously, when a DM conversation contained only messages from the authenticated user (i.e., the user sent a message but received no reply within the conversation window), the conversation would be named with a generic identifier like dm-unknown-1 instead of showing the actual target recipient's name. This made it difficult to identify which person the conversation was with.

Root Cause

The issue occurred in the build_dm_conversation function, which determined conversation participants solely from message senders. When only the authenticated user had sent messages, the function couldn't identify the "other participant" and defaulted to using "unknown" in the conversation ID.

Solution

This PR enhances the DM conversation building logic to fetch room membership information from the Webex API when message senders alone don't provide sufficient participant information.

Changes Made

Core Logic Enhancement (summarizer/grouping.py)

• Enhanced build_dm_conversation function: Added optional WebexAPI client parameter and fallback logic to fetch room memberships when the other participant cannot be identified from messages
• Updated function signatures: Modified group_dm_conversations and group_all_conversations to accept and pass through the WebexAPI client
• Added error handling: Gracefully handles API failures when fetching room memberships with appropriate logging

WebexAPI Client Access (summarizer/webex.py)

• Added client property: Exposed the underlying WebexAPI instance through a public property on WebexClient for use in conversation grouping logic

Integration (summarizer/runner.py)

• Updated function call: Modified the call to group_all_conversations to pass the WebexAPI client instance

Type Safety (webexpythonsdk-stubs/)

• Added Memberships API: Extended WebexAPI stub with memberships property and MembershipsAPI class
• Added Membership model: Created Membership class stub with required properties (id, roomId, personId, personEmail, isModerator)

How It Works

1. Primary approach: Attempts to identify the other participant from message senders (existing behavior)
2. Fallback approach: When that fails, queries the Webex API's memberships endpoint for the room
3. Participant resolution: Finds the membership record for a person who isn't the authenticated user
4. Name resolution: Uses the existing safe_get_person function to get the person's display name
5. Graceful degradation: Falls back to "unknown" if API calls fail, maintaining backward compatibility

Results

After this fix, DM conversations now display meaningful names even when only the authenticated user has sent messages:

Before:

dm-unknown-1 | 1 messages | Participants: Christopher Hart | ...

After:

dm-andrea-testino-15 | 1 messages | Participants: Christopher Hart | ...

Testing

• ✅ Verified fix works with the original reproduction case
• ✅ Confirmed backward compatibility when both participants have sent messages
• ✅ Tested graceful degradation when API calls fail
• ✅ No linting errors introduced

Impact

• User Experience: Significantly improves conversation identification in the daily summary
• Debugging: Makes it easier to understand communication patterns and follow up on unanswered messages
• Backward Compatibility: Maintains all existing functionality while adding the enhancement
• Performance: Minimal impact - API calls only made when necessary (fallback scenario)