MCP Registry remote URL variables are not converted during install

## Type of issue

Bug

## Description

VS Code accepts MCP Registry manifests using the `2025-12-11` `server.json` schema, but does not convert variables declared on a remote transport when generating `mcp.json`.

The schema permits remote URLs such as:

```json
{
"type": "streamable-http",
"url": "https://{environment_id}.example.com/mcp",
"variables": {
"environment_id": {
"description": "Environment ID",
"isRequired": true,
"format": "string"
}
}
}
```

VS Code processes variables declared in headers and arguments, but copies `remotes[].url` unchanged and drops `remotes[].variables`. The installed server consequently has an unresolved hostname and no input through which the user can provide the environment ID.

This affects multi-tenant remote MCP servers whose tenant, environment, region, or deployment identifier forms part of the endpoint URL. The [Dynatrace MCP server manifest](https://github.com/Dynatrace/dynatrace-for-ai/blob/main/server.json) is one concrete example.

## Steps to reproduce

1. Use a `server.json` manifest with the `2025-12-11` schema and a `remotes[]` entry containing a URL placeholder plus a matching `variables` declaration.
2. Run **MCP: Install Server from Manifest...** and select the manifest, or install the same entry from an MCP Registry.
3. Run **MCP: Open User Configuration**.
4. Inspect the generated server URL and top-level `inputs` array.

## Actual behavior

The generated configuration keeps the Registry placeholder literal:

```jsonc
"url": "https://{environment_id}.example.com/mcp"
```

No `environment_id` entry is added to `inputs`. Starting the server therefore uses an unresolved/literal hostname rather than prompting the user.

## Expected behavior

VS Code should convert declared Registry URL placeholders into its configuration-input syntax:

```jsonc
"url": "https://${input:environment_id}.example.com/mcp"
```

It should also create the corresponding top-level input using the description, secret flag, default, and choices from the Registry declaration. Explicit fixed values should be substituted directly rather than exposed as prompts.

## Schema references

- Published [`2025-12-11` JSON Schema](https://static.modelcontextprotocol.io/schemas/2025-12-11/server.schema.json)
- [`RemoteTransport.variables`](https://github.com/modelcontextprotocol/registry/blob/main/docs/reference/api/openapi.yaml#L900-L912)
- [`ServerDetail.remotes[]` references `RemoteTransport`](https://github.com/modelcontextprotocol/registry/blob/main/docs/reference/api/openapi.yaml#L999-L1002)
- Feature introduction: [modelcontextprotocol/registry#570](https://github.com/modelcontextprotocol/registry/pull/570)
- Schema release: [modelcontextprotocol/registry#841](https://github.com/modelcontextprotocol/registry/pull/841)

## Additional considerations

- Support both streamable HTTP and SSE remotes.
- Replace repeated occurrences of a declared placeholder while leaving undeclared placeholders unchanged.
- Deduplicate compatible variables shared between URLs and headers.
- Do not silently discard conflicting defaults, choices, or secret metadata for the same input ID.
- Preserve registry-only access checks by treating the Registry and generated VS Code template forms as equivalent, without introducing wildcard URL matching.