fix: StackOverflowError caused by Jackson serialization circular dependency in remote mode

## Bug Description

When using sandbox in **remote mode** (via `RemoteHttpClient`), calling `SandboxService.getInfo(sandbox)` or any other `@RemoteWrapper` method results in a `StackOverflowError` due to Jackson
serialization triggering an infinite recursive loop.

## Root Cause Analysis

The call chain:

SandboxService.getInfo(sandbox) # line 539
→ mapper.writeValueAsString(sandbox) # serializes entire Sandbox
→ Jackson traverses getters, finds getDesktopUrl()
→ FilesystemSandbox.getDesktopUrl() # line 84
→ BrowserSandbox.getDesktopUrl() # line 83
→ GuiSandbox.getDesktopUrl() # line 88
→ GuiMixin.getDesktopUrl(managerApi, ...) # line 47
→ managerApi.getInfo(sandbox) # calls back to getInfo()
→ mapper.writeValueAsString(sandbox) # infinite loop → StackOverflowError

### Key Observations

1. `Sandbox.getInfo()` (line 153) has `@JsonIgnore` and is correctly excluded from serialization
2. `getDesktopUrl()` in `FilesystemSandbox`, `BrowserSandbox`, and `GuiSandbox` has **no** `@JsonIgnore`, so Jackson invokes it during serialization
3. `getDesktopUrl()` → `GuiMixin.getDesktopUrl()` calls `managerApi.getInfo(sandbox)`, which in remote mode calls `writeValueAsString(sandbox)` again — closing the loop
4. `SandboxService.getInfo()` in remote mode serializes the entire `Sandbox` object, when it only needs `sandboxId` (same as local mode: `sandboxMap.getSandbox(sandbox.getSandboxId())`)

### Affected Methods in `SandboxService.java`

All `@RemoteWrapper` methods that call `mapper.writeValueAsString(sandbox)`:

| Method | Line |
|--------|------|
| `createContainer(Sandbox)` | ~153 |
| `getInfo(Sandbox)` | ~539 |
| `listTools(Sandbox, ...)` | ~602 |
| `callTool(Sandbox, ...)` | ~633 |
| `addMcpServers(Sandbox, ...)` | ~663 |

### Affected Files

- `sandbox-core/.../box/FilesystemSandbox.java` — line 84
- `sandbox-core/.../box/BrowserSandbox.java` — line 83
- `sandbox-core/.../box/GuiSandbox.java` — line 88
- `sandbox-core/.../box/GuiMixin.java` — line 47
- `sandbox-core/.../manager/SandboxService.java` — lines 153, 539, 602, 633, 663

## Suggested Fix

In `SandboxService` remote mode methods, only serialize `sandboxId` instead of the entire `Sandbox` object. The remote API likely only needs the sandbox ID for container lookups, same as
local mode (`sandboxMap.getSandbox(sandbox.getSandboxId())`).

For example, in `getInfo()`:

```java
// Before (line 539):
String sandboxJson = mapper.writeValueAsString(sandbox);

// After:
String sandboxJson = mapper.writeValueAsString(Map.of("sandboxId", sandbox.getSandboxId()));

Steps to Reproduce

1. Configure sandbox to run in remote mode (with RemoteHttpClient)
2. Call any sandbox method that internally triggers getDesktopUrl() (e.g., sandbox.getInfo())
3. Observe StackOverflowError