Custom Headers Not Applied in Azure Function App Settings

### Is your question related to a specific version? If so, please specify:
Azure Functions (linux) - Runtime version: `4.636.0.0`

### What language does your question apply to? (e.g. C#, JavaScript, Java, All)
We're using C# - although this may apply to All languages as it relates to the configuration of HTTP headers using `host.json` and Application Settings.

### Question
I am trying to [override `host.json values](https://learn.microsoft.com/en-us/azure/azure-functions/functions-host-json#override-hostjson-values) configure custom HTTP headers for my Azure Function App via app settings, but I am not seeing the headers in the HTTP response. I have attempted the following approaches:
1. Adding a custom header `XCustomHeader` using the key `AzureFunctionsJobHost__extensions__http__customHeaders_XCustomHeader` with a value of `TestHeaderValue`, but this header is not present in the response.

2. Using key `AzureFunctionsJobHost__extensions__http__customHeaders` with an escaped JSON as the value

Despite these efforts, none of the custom headers are reflected in the response.

Is there a limitation in how the Azure Functions platform processes these headers when defined in app settings?

### Steps to reproduce
1. Add the following app setting in the Azure Portal using **Advanced Edit**:
- Method 1:
```json
{
"name": "AzureFunctionsJobHost__extensions__http__customHeaders_XCustomHeader",
"value": "TestHeaderValue",
"slotSetting": false
}
```
- Method 2:
```json
{
"name": "AzureFunctionsJobHost__extensions__http__customHeaders",
"value": "{\"Content-Security-Policy\": \"default-src 'self';\", \"Permissions-Policy\": \"accelerometer=(), autoplay=();\"}",
"slotSetting": false
}
```
2. Save the configuration and restart the Function App.
3. Navigate to the function app

#### Expected behavior
The custom headers (XCustomHeader, Content-Security-Policy, etc.) should be present in the HTTP response.

#### Actual behavior
None of the custom headers are applied to the response

Start with the linked host.json override documentation and compare the two Azure portal app-setting formats described in the reproduction steps. Trace how the Azure Functions host processes customHeaders from application settings; done means establishing whether the configuration is supported and documenting or correcting the behavior so the expected response headers are applied.

Written by the indexing model from the issue text.