feat: Adding WhatsApp BSUID support to Messages

[superchilled]

This PR updates the Messages package to add support for sending WhatsApp messages using a BSUID in the to field. Specifically it:

• Adds a unit test to cover setting a BSUID in the to field of a WhatsApp message object
• Updates the BaseWhatsapp model to allow the setting of the BSUID in the to field

[Copilot]

Pull request overview

This PR updates the Messages WhatsApp models to support using a WhatsApp BSUID as the to recipient identifier (in addition to a phone number), and adds a unit test to cover that scenario.

Changes:

• Adds a unit test asserting a WhatsApp text message can be serialized with a BSUID in to.
• Updates the WhatsApp base model to widen the to field type to accept BSUID-style identifiers.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 2 comments.

File	Description
messages/tests/test_whatsapp_models.py	Adds a new unit test for serializing a WhatsApp text message where to is a BSUID.
messages/src/vonage_messages/models/whatsapp.py	Updates BaseWhatsapp.to typing/docs to allow BSUID usage for WhatsApp recipients.

Suppressed comments (1)

messages/src/vonage_messages/models/whatsapp.py:46

• to is now Union[PhoneNumber, str], but PhoneNumber is already an Annotated[str, Field(pattern=...)]. Adding a plain str to the union makes validation effectively always succeed (any string matches the str branch), so invalid recipients that were previously rejected will now be accepted.

    to: Union[PhoneNumber, str]

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