az containerapp update resets template.scale.cooldownPeriod to 300

### Describe the bug

Our Container App is provisioned with Terraform using
`cooldown_period_in_seconds = 3600`. The deployed app initially reports
`properties.template.scale.cooldownPeriod` as `3600`.

After we run `az containerapp update` to deploy a new image, the value changes
to the default of `300`, even though the update command does not specify a
cooldown period.

### Related command

az containerapp update -g -n \
--image /:

### Errors

/

### Issue script & Debug output

/

### Expected behavior

/

### Environment Summary

azure-cli 2.88.0 *

core 2.88.0 *
telemetry 1.1.0

Dependencies:
msal 1.36.0
azure-mgmt-resource 24.0.0

Python location '/usr/bin/python3.12'
Config directory '/root/.azure'
Extensions directory '/root/.azure/cliextensions'

Python (Linux) 3.12.9 (main, Jun 2 2026, 15:09:20) [GCC 13.2.0]

Legal docs and information: aka.ms/AzureCliLegal

### Additional context

_No response_

Start from the `az containerapp update` command in the Container App extension and reproduce the image update with a non-default `cooldownPeriod`. Inspect the update payload handling for `properties.template.scale`, then verify that an image-only update preserves the existing cooldown value.

Written by the indexing model from the issue text.