fix(pi): preserve tool call arguments

[drewstone]

Pi raw traces store tool inputs under the arguments field. The adapter only read input and args, so every Pi tool call appeared to have no input.

This maps the existing field through the existing canonical serializer and adds a realistic regression case.

Proof:

• Real corpus: 1,420 sessions parsed; 79,243/79,243 tool calls captured after versus 0/79,243 before
• Full tests: 369/369 pass
• Typecheck passes
• Clean merge against current origin/main

A separate pre-existing Pi tool-result association defect is intentionally excluded from this focused change.