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#### What I did
This PR updates the get_gnmi_port() function in the smartswitch reboot helper script to align with recent YANG model changes for DPU configuration. The function now retrieves the GNMI port from the updated CONFIG_DB schema.

https://github.com/sonic-net/sonic-buildimage/blob/master/src/sonic-yang-models/doc/Configuration.md#dpu-configuration

#### How I did it
- Modified database key pattern from DPU_PORT|$DPU_NAME to DPU|*$DPU_NAME
- Added default parameter value for DPU_NAME
- Updated query method to iterate through matching keys

#### How to verify it
Run the reboot command with latest configuration changes.

#### Previous command output (if the output of a command-line utility has changed)

#### New command output (if the output of a command-line utility has changed)