Context

ToolCallDag::build() extracts leaf string values from each call's input JSON. The dependency check in native.rs (~line 633) calls extract_string_values() again per-tool in the tier dispatch loop. For n≤10 calls this is negligible, but it is a DRY violation.

Fix

Cache the extracted string vec alongside each node in ToolCallDag, expose it via a method, and remove the redundant call in native.rs.

Identified by: performance agent during PR #1712 review (PERF-02)