Support variable interpolation in `chatLanguageModels.json` requestHeaders

## Feature request

Support variable (template) interpolation in `requestHeaders` values of `chatLanguageModels.json`, so values can be dynamic per workspace — e.g. include the current repo/folder name in a custom header without editing the file per repo.

## Motivation / Use case

With the Custom Endpoint (`vendor: customendpoint`) provider, I configure a gateway (LiteLLM) and want to tag every request with the repository it originates from:

```json
{
"name": "Example Vendor",
"vendor": "customendpoint",
"apiKey": "${input:chat.lm.secret.lalala}",
"models": [
{
"id": "example/model",
"url": "https://litellm.example.com",
"requestHeaders": {
"x-litellm-tags": "${workspaceFolderBasename}"
}
}
]
}
```

I want `x-litellm-tags` to resolve to the current repo name per workspace.

## Current behavior

`requestHeaders` values are sent verbatim. I traced the path:

- `languageModelsConfigurationService.ts` parses the JSON with a `JSONVisitor` — values taken literally.
- `languageModels.ts` `_resolveConfiguration()` only resolves `${input:...}` for properties marked `secret: true` in the schema (e.g. `apiKey`).
- `customEndpointProvider.ts` copies `requestHeaders` verbatim into model info.
- `openAIEndpoint.ts` `_sanitizeCustomHeaders()` filters names but does not interpolate values.
- Only `${apiKey}` is specially replaced (`_interpolateApiKey`).

So `${workspaceFolder}`, `${workspaceFolderBasename}`, `${env:FOO}`, `${userHome}` etc. are sent as literal strings.

## Proposed change

Apply the same variable substitution already used by other config surfaces (tasks.json/launch.json via `IConfigurationResolverService`) to `requestHeaders` values — or at minimum support `${workspaceFolder}`, `${workspaceFolderBasename}`, `${userHome}`, and `${env:VAR}`.

At minimum a subset could be resolved: identical tokens to `${apiKey}` but scoped to workspace/user/env.

## Notes

- macOS + VS Code 1.x, Dev Container remote extension host in use.
- This is the `chatLanguageModels.json` BYOK custom-endpoint schema (`requestHeaders`), not the deprecated `chat.customOAIModels` setting.