Fix Windows ARM64 managed Node provisioning + preserve agent error data

[iroiro147]

Fix Windows ARM64 managed Node provisioning + preserve agent error data

Summary

Fix two compounding defects that break Windows ARM64 hosts running the x64 Buzz desktop build under emulation (issue #4069):

1. Runtime architecture detection for managed Node provisioning: Previously, managed_node_paths.rs and agent_discovery/managed_node.rs used std::env::consts::ARCH (a compile-time constant) to select the Node platform. An x64-built Buzz binary running under Windows x64 emulation on ARM64 hardware always reports x86_64, causing it to download and install win-x64 Node. The agent adapter's npm packages (installed under the system ARM64 Node) were therefore mismatched, and the claude.exe launch failed with binary-not-found errors.

2. Preserve JSON-RPC error.data in agent_error_from_json: When the adapter returned {code, message, data}, the data payload was silently dropped if message was a string, hiding the actionable diagnostic ("Claude native binary not found for win32-x64...") from the user.

Changes

• desktop/src-tauri/src/managed_agents/managed_node_paths.rs: On Windows, resolve the actual machine architecture at runtime via IsWow64Process2; fall back to compile-time arch if the API is unavailable (e.g., pre-Windows 10 1709) or fails. Add unit tests covering the runtime resolver and the artifact-selection path. Non-Windows behavior is unchanged (compile-time arch remains authoritative).
• desktop/src-tauri/src/commands/agent_discovery/managed_node.rs: Replace the compile-time MANAGED_NODE_ARTIFACT constant with a runtime resolver managed_node_artifact() that selects the correct Windows artifact (win-x64 or win-arm64) based on the actual machine architecture, matching the behavior in managed_node_paths.rs.
• crates/buzz-acp/src/acp.rs: Append the serialized error.data payload to the error message when both message and data fields are present in a JSON-RPC error.

Testing

• Added tests:
  • managed_node_paths.rs::target_arch_returns_compile_time_value_on_non_windows (verifies non-Windows platforms defer to compile-time arch)
  • managed_node_paths.rs::target_arch_returns_known_value_on_windows (verifies Windows runtime detection returns a valid arch)
  • agent_discovery/managed_node.rs::managed_node_artifact_returns_artifact_matching_runtime_platform (verifies the artifact resolver selects the correct platform string)
  • acp.rs::agent_error_from_json_appends_data_when_message_and_data_present (verifies error.data is preserved in the error message)
• All existing tests pass:
  • buzz-desktop lib: 2090 passed, 0 failed
  • buzz-acp lib: 662 passed, 0 failed
  • Clippy: clean for both crates

Compatibility

• No breaking changes to APIs or file formats.
• Non-Windows platforms: no behavior change (compile-time arch remains authoritative).
• Windows x64 native: no behavior change (runtime detection returns x86_64).
• Windows ARM64: fixes managed Node provisioning; existing win-x64 installs remain untouched (no forced migration).

Fixes #4069