Reject connection secrets without a connection type

[henry3260]

Why

Connection secrets in JSON format could omit conn_type. Although the secrets backend deserialized those values, the Execution API requires a connection type and the task could ultimately receive a misleading “connection not found” error.

This addresses the conn_type portion of related: #57864.

What

• Reject JSON connection secrets whose conn_type is missing, null, empty, or whitespace-only.
• Raise a clear ValueError before constructing an incomplete Connection.
• Add regression coverage for each invalid conn_type value.

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