fix(sandbox): serialize only sandboxId to prevent Jackson circular dependency

[itxaiohanglover]

Fixes #105

Problem

SandboxService remote mode methods call mapper.writeValueAsString(sandbox), which triggers Jackson to traverse all getters including getDesktopUrl(). In remote mode, getDesktopUrl() calls managerApi.getInfo(sandbox), which calls writeValueAsString(sandbox) again — creating an infinite recursion that results in StackOverflowError.

What changed

Replaced all 5 writeValueAsString(sandbox) calls with writeValueAsString(Map.of("sandboxId", sandbox.getSandboxId())):

• createContainer(Sandbox) — line 153
• getInfo(Sandbox) — line 539
• listTools(Sandbox, ...) — line 602
• callTool(Sandbox, ...) — line 633
• addMcpServers(Sandbox, ...) — line 663

The remote API only needs the sandbox ID for container lookups — same as local mode (sandboxMap.getSandbox(sandbox.getSandboxId())).

Validation

• No behavior change in local mode (remoteHttpClient is null)
• Remote mode no longer serializes the full Sandbox object, breaking the circular dependency
• @JsonIgnore on getInfo() was already correct — the issue was getDesktopUrl() lacking it
• This fix is more robust than adding @JsonIgnore to getDesktopUrl() because it prevents any future getter from causing similar issues

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