json: raise wuffs cursor nesting bound to 16

[botengyao]

Commit Message: json: raise wuffs cursor nesting bound to 16

Additional Description:

This raises kMaxTrackedDepth to 17 (effective limit 16), leaving headroom for
nested tool-call arguments, and records the Gemini path in the rationale
comment. Before: nesting at depth >= 9 fails with InvalidArgumentError. After:
depth >= 17 fails. The per-depth tracking arrays are fixed-size and grow with
the constant; the existing TODO about moving to dynamic depth is unchanged.

Risk Level: Low — one compile-time bound is raised, no logic changes.
Per-cursor fixed state grows by 8 array slots.

Testing: Updated the cursor depth boundary tests (16 accepted, 17 rejected) and
added a parser test that a realistic Gemini functionCall.args document parses.

Docs Changes: N/A
Release Notes: N/A
Platform Specific Features: N/A

[repokitteh-read-only]

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🐱

Caused by: #46933 was opened by botengyao.

see: more, trace.