fix(pi-sdk): Fix nesting of child spans in tool spans

[Aaron (btaaron)]

Unit flagged that Pi creates an automatic tool span, but currentSpan() inside the tool still returns the outer span, meaning nested spans get the wrong parent. This fix binds the existing tool span while its executor runs, preserving caller context and executor's return and error behavior. Fixes SDK-389

Tested

• Identical harness reproduces incorrect parenting in 9/9 cases on Braintrust 3.30.0 and passes 9/9 against package built from 80af362c. All 53 context observations use correct tool span after fix. Covers overlapping sessions with reused call IDs, session reuse, nested traces, sync throw, async rejection, caller restoration
• 14 new regression cases pass, covering both Pi stream properties, concurrent tools/prompts, executor semantics, missing/stale state, and instrumentation suppression
• All 24 existing Pi end-to-end tests pass. SDK suite passes on Ubuntu and Windows with Node 20, 22, 24, and 26 (1911 SDK tests plus 61 automatic-instrumentation tests; 3 existing skips)