[#125]: Fix Codex v0.139.0 compat — accept JSON-object function_call arguments and add oneOf/allOf schema tests

[delexw]

Summary

Codex v0.139.0 (PRs #24118, #27084) changed tool and connector input schemas to preserve oneOf and allOf structures instead of flattening them. Large schemas also keep more shallow structure when compacted.

Root cause: When a tool's schema uses composition keywords, the model may generate function_call arguments as a JSON object directly in the JSONL payload rather than as a stringified JSON string. The existing handler used .as_str().unwrap_or("{}"), which silently dropped any non-string arguments value (producing Value::Null on the ToolCall). The mcp_tool_call handler already handled both forms correctly.

Changes

Fix (src-tauri/src/parser/turn.rs)

Updated the function_call response_item handler to accept both string and object argument formats, matching the existing mcp_tool_call handler:

// Before
let arguments_str = payload.get("arguments").and_then(|v| v.as_str()).unwrap_or("{}");

// After
let arguments_str = match payload.get("arguments") {
    Some(Value::String(s)) => s.clone(),
    Some(v) => serde_json::to_string(v).unwrap_or_else(|_| "{}".to_string()),
    None => "{}".to_string(),
};

Regression tests (9 new tests across 3 files)

entry.rs (5 tests): All four standard v0.139.0 entry types parse correctly; function_call with JSON-object arguments is preserved; mcp_tool_call with oneOf/allOf schema fields is handled; session_meta with tools having oneOf, allOf, and deeply nested composition schemas all parse without panic.

session.rs (3 tests): Full parse_session round-trips with tools having oneOf schemas, allOf connector schemas, and a function_call made in a session whose session_meta carries complex tool schemas — confirms session id, cli_version, turn count, and tool call classification are all correct.

turn.rs (5 tests): JSON-object function_call arguments are preserved through the full build_turns pipeline; complex nested argument structures are not dropped; mcp_tool_call items with oneOf/allOf input_schema fields are classified as McpTool; function_call in a session with complex session_meta tool schemas is classified correctly.

Verification

• cargo test --lib: 220 tests pass (215 before this PR)
• cargo clippy -- -D warnings: clean
• cargo fmt --check: clean
• HTTP API smoke test (--headless mode): POST /api/sessions returns JSON array

Fixes #125