fix(agents): estimate the tools breakdown from serialized schemas [review #4596]

`approxTokens(opts.tools)` hit approxTokens' array branch, which charges a flat
~64 per non-text block — so the "Tools" segment was ~64×toolCount regardless of
how large each tool's name/description/parameter schema actually is, and the
shortfall silently inflated the "Messages" remainder. Serialize the tool array
first so the estimate reflects what really occupies the prompt.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: kevin-dp

packages/agents-runtime/src/pi-adapter.ts

@@ -320,7 +320,12 @@ export function createPiAgentAdapter(
     // total minus these (see token-accountant computeContextBreakdown).
     const tokenBreakdown = {
       system: approxTokens(opts.systemPrompt),
-      tools: approxTokens(opts.tools),
+      // Serialize first: approxTokens' array branch charges a flat ~64 per
+      // non-text block, so passing the AgentTool[] directly would estimate
+      // ~64×toolCount regardless of the real schema size. JSON.stringify
+      // captures each tool's name + description + parameter schema (functions
+      // drop out), which is what actually occupies the prompt.
+      tools: approxTokens(JSON.stringify(opts.tools)),
     }
 
     const transformContext =