similiar to: microsoft/teams.net#198
fixes: #406

---------

Co-authored-by: lilydu <[email protected]>

similiar to: microsoft/teams.net#198 and
microsoft/teams.ts#434

fixes: #245

---------

Co-authored-by: lilydu <[email protected]>

…icrosoft#198)

Teams sends meeting start/end events with PascalCase properties in the
`value` object (`Id`, `MeetingType`, `Title`, `JoinUrl`, `StartTime`,
`EndTime`), causing deserialization failures.

### Changes
- Updated `MeetingStartActivityValue` and `MeetingEndActivityValue` to
use PascalCase in `JsonPropertyName` attributes
- Updated test fixtures to match actual Teams payload format
- Added comprehensive tests validating serialization, deserialization,
and polymorphic type handling:
  - Basic deserialization with Teams PascalCase payloads
  - Round-trip serialization/deserialization verification
  - Polymorphic deserialization as `EventActivity` base class
  - Polymorphic deserialization as `IActivity` interface

### Example
```json
{
  "name": "application/vnd.microsoft.meetingEnd",
  "type": "event",
  "value": {
    "Id": "MCMx...",           // Was: "id"
    "MeetingType": "Scheduled", // Was: "meetingType"
    "Title": "Daily Standup",   // Was: "title"
    "JoinUrl": "https://...",   // Was: "joinUrl"
    "EndTime": "2025-10-31..."  // Was: "endTime"
  }
}
```

### Testing
- ✅ All 339 tests pass
- ✅ 10 comprehensive PascalCase-specific tests added covering all
scenarios
- ✅ Serialization produces correct PascalCase format
- ✅ Round-trip serialization/deserialization verified

This deviates from the library's camelCase convention but matches the
actual Teams platform format.

<!-- START COPILOT CODING AGENT SUFFIX -->



<details>

<summary>Original prompt</summary>

> 
> ----
> 
> *This section details on the original issue you should resolve*
> 
> <issue_title>Exception while deserializing Activity when meeting
starts or ends - because of case sensitivity</issue_title>
> <issue_description>When meeting starts or ends, library crashes while
deserializing JSON to activity. Deep inside Activity Json Converter.
> I get this JSON from Teams:
> 
> ```
> {
>     "name": "application/vnd.microsoft.meetingEnd",
>     "type": "event",
>     "timestamp": "2025-10-31T11:38:15.5375726Z",
>     "id": "1761910695513",
>     "channelId": "msteams",
> "serviceUrl":
"https://smba.trafficmanager.net/emea/167c22a9-1b2e-439c-ad74-cc77e9e118d8/",
>     "from": {
> "id":
"29:1geTNfcvfJus0De5z4gr7HeHGMOuln9LY8aHFGtwBqhOl7ZYQFcM2CL1ODjhgHE1XTq3vBeeRlGGGPvFWi0BzRw",
>         "name": "",
>         "aadObjectId": "86a23cfc-f78e-424a-8947-7ae0ce242da1"
>     },
>     "conversation": {
>         "isGroup": true,
>         "conversationType": "groupChat",
>         "tenantId": "167c22a9-1b2e-439c-ad74-cc77e9e118d8",
> "id":
"19:meeting_MTRmMTQ5NDYtMTYyYi00NmNlLWI4ZTQtN2I1MTYzM2RkYTg3@thread.v2"
>     },
>     "recipient": {
>         "id": "28:c9a052ed-f68c-4227-b081-01da0669c49c",
>         "name": "teams-bot"
>     },
>     "entities": [
>         {
>             "locale": "en-US",
>             "country": "US",
>             "type": "clientInfo"
>         }
>     ],
>     "channelData": {
>         "tenant": {
>             "id": "167c22a9-1b2e-439c-ad74-cc77e9e118d8"
>         },
>         "source": null,
>         "meeting": {
> "id":
"MCMxOTptZWV0aW5nX01UUm1NVFE1TkRZdE1UWXlZaTAwTm1ObExXSTRaVFF0TjJJMU1UWXpNMlJrWVRnM0B0aHJlYWQudjIjMA=="
>         }
>     },
>     "value": {
>         "MeetingType": "Scheduled",
>         "Title": "asdasd",
> "Id":
"MCMxOTptZWV0aW5nX01UUm1NVFE1TkRZdE1UWXlZaTAwTm1ObExXSTRaVFF0TjJJMU1UWXpNMlJrWVRnM0B0aHJlYWQudjIjMA==",
> "JoinUrl":
"https://teams.microsoft.com/l/meetup-join/19%3ameeting_MTRmMTQ5NDYtMTYyYi00NmNlLWI4ZTQtN2I1MTYzM2RkYTg3%40thread.v2/0?context=%7b%22Tid%22%3a%22167c22a9-1b2e-439c-ad74-cc77e9e118d8%22%2c%22Oid%22%3a%2286a23cfc-f78e-424a-8947-7ae0ce242da1%22%7d",
>         "EndTime": "2025-10-31T11:38:15.5375726Z"
>     },
>     "locale": "en-US"
> }
> ```
> 
> Look at casing. Every property is in lower case BUT **value** object
properties which are in upper case.
> 
> Thus JsonConverter throws an error:
> 
> `'JSON deserialization for type
'Microsoft.Teams.Api.Activities.Events.MeetingEndActivityValue' was
missing required properties including: 'id', 'meetingType', 'joinUrl',
'title', 'endTime'.'`
> 
> It's completely blockers. Do you have any fast workaround?
> </issue_description>
> 
> <agent_instructions>update the meetings types in the
`Microsoft.Teams.Api` package to be PascalCase instead of CamelCase when
serializing/deserializing to/from JSON (ie via
`JsonPropertyNameAttribute`).</agent_instructions>
> 
> ## Comments on the Issue (you are @copilot in this section)
> 
> <comments>
> <comment_new><author>@aacebo</author><body>
> thanks for the report, I'm reaching out to our platform team to figure
out why they are sending the payload in this format, to our knowledge
this is the incorrect format using pascal case instead of camel
case.</body></comment_new>
> </comments>
> 


</details>

- Fixes microsoft#196

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---------

Co-authored-by: copilot-swe-agent[bot] <[email protected]>
Co-authored-by: aacebo <[email protected]>
Co-authored-by: Alex Acebo <[email protected]>
Co-authored-by: Lily Du <[email protected]>
Co-authored-by: lilydu <[email protected]>