[Teams] Error 400 in sending message

### One-line summary

trying to send message with teams connector ALWAYS fails due to missing recipient

### Area of the portal

Connectors

### Severity

Medium (workaround exists, but annoying)

### What happened?

We tried to use the Teams connector to send a Teams message to an user.
Every time we call the connector with the field to filled with the mail, it fails with

error": "POST https://REDACTED.common.logic-westcentralus.azure-apihub.net/apim/teams/REDACTED/v1.0/teams/conversation/gatherinput/poster/Flow%20bot/location/Chat%20with%20Flow%20bot/$subscriptions failed with status 400: {\r\n \"error\": {\r\n \"code\": 400,\r\n \"message\": \"**### _The recipient is missing_**\",\r\n \"source\": \"REDACTED.common.logic-westcentralus.azure-apihub.net\",\r\n \"path\": \"choose[20]\\\\when[1]\",\r\n \"policyId\": \"\"\r\n }\r\n}

### What did you expect to happen?

To receive a message on teams

### Steps to reproduce

1. from azure.connectors.teams import PostCardAndWaitForResponseInput
2. return PostCardAndWaitForResponseInput(
notification_url=notification_url,
body=body,
)

3. body = {
"to": recipient,
"messageBody": json.dumps(card, separators=(",", ":")),
"isAlert": False,
}

with these we debug and we see that is sent the correct recipient

### Page URL

https://connectors.azure.com/:subscriptionId/:resourceGroup/:resourceName/connections

### App version / build / commit SHA

c5965f98

### Browser & version (User-Agent)

Mozilla/5.0 (Macintosh; Intel Mac OS X 10_15_7) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/150.0.0.0 Safari/537.36

### Viewport size & theme

1439x781 @ dark

### Entra tenant ID

8c586c92-cabd-4ce5-983a-be9869cdc625

### Azure subscription ID

651f7c62-50f2-474d-a2aa-a004377ff719

### Anything else

_No response_

Start with the reported reproduction using azure.connectors.teams.PostCardAndWaitForResponseInput and inspect how notification_url and the body containing to and messageBody are sent. Reproduce the 400 response, then verify whether the recipient reaches the Teams connector; done means the connector accepts the recipient and delivers the Teams message.

Written by the indexing model from the issue text.