fix(acp): preserve JSON-RPC error.data when message is present (#4069)

[iroiro147]

Summary

Fixes #4069 (partial — covers the cross-platform observability defect; the Windows ARM64 Node provisioning half is left to a follow-up).

agent_error_from_json in crates/buzz-acp/src/acp.rs was discarding the JSON-RPC error.data payload whenever the agent returned a string message. The ACP TypeScript SDK wraps plain thrown exceptions as:

{"code": -32603, "message": "Internal error", "data": {"details": "<fully actionable adapter message>"}}

so the actionable adapter text never reached Buzz logs — operators saw only Agent reported error (code -32603): Internal error.

What changed

When message is a string and a non-null data field is present, append a compact form of data to the surfaced message:

• Prefer a nested data.details string when present (matches the ACP TS SDK's wrap shape).
• Otherwise fall back to the data value stringified (handles plain string and structured data cases).
• Truncate defensively at 2048 chars so a pathological adapter cannot flood logs.
• data: null is ignored (no information).
• Errors without a string message still fall back to the full JSON object as before — no behavior change there.

Test plan

Five new unit tests in crates/buzz-acp/src/acp.rs::tests:

• agent_error_from_json_appends_data_details_when_present — the Windows ARM64 shape from Windows on ARM64: Claude managed agents fail every session/new with -32603 (win-x64 Node provisioned on ARM64 host; error.data dropped from logs) #4069: Internal error + data.details with the actionable remediation. Asserts both halves are surfaced.
• agent_error_from_json_appends_string_data_when_details_missing — plain string data.
• agent_error_from_json_ignores_null_data — data: null produces the original message unchanged.
• agent_error_from_json_truncates_oversized_data — 4096-char data is truncated with ellipsis.
• Existing agent_error_from_json_uses_message_field_when_present (no data key) — unchanged.

Verification:

cargo check -p buzz-acp          # clean
cargo clippy -p buzz-acp --all-targets -- -D warnings   # clean
cargo test -p buzz-acp --lib     # 665 passed, 0 failed
cargo test -p buzz-acp agent_error_from_json   # 6 passed

Notes

The Windows ARM64 side of #4069 (Buzz provisioning win-x64 Node on an ARM64 host because managed_node_paths.rs reads std::env::consts::ARCH at compile time) is not addressed here — that's a runtime detection change requiring IsWow64Process2 / PROCESSOR_ARCHITEW6432 and would benefit from maintainer input on supported host matrix. This PR ships just the cross-platform logging fix so the next adapter failure of this shape surfaces its own remedy in the log.

Closes #4069