Apply review feedback: trace panic safety, cost pre-check, parallel JoinSet, ToolFuture alias

[Copilot]

Four targeted fixes addressing reviewer feedback on the AI-native runtime crates.

rustapi-context — trace.rs

• SpanGuard::drop: Clarified comment — this is a panic-safety path, not a general "forgot to call complete" warning. Explicit callers use mem::forget to skip it; the Drop impl only fires on unwinding.

rustapi-context — cost.rs

• CostTracker::record: Switched from fail-open (apply → check) to pre-check (check projected totals → apply only if within limits). A rejected call no longer mutates any atomic counters. Added a doc-comment concurrency note explaining the intentional TOCTOU trade-off in the lock-free design.

rustapi-tools — graph.rs

• ToolNode::Parallel: Replaced Vec<JoinHandle> + sequential in-order awaiting with tokio::task::JoinSet. Results are collected as tasks complete rather than in submission order; a first failure immediately calls set.abort_all().

rustapi-tools — tool.rs

• ClosureTool: Added a ToolFuture<'a> type alias for the full Pin<Box<dyn Future<Output = Result<ToolOutput, ToolError>> + Send + 'a>> type. All ClosureTool bounds now use the alias:

// Before
F: Fn(&RequestContext, serde_json::Value)
    -> std::pin::Pin<Box<dyn std::future::Future<Output = Result<ToolOutput, ToolError>> + Send + '_>>
    + Send + Sync + 'static

// After
F: Fn(&RequestContext, serde_json::Value) -> ToolFuture<'_> + Send + Sync + 'static

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