strict_schema: recurse into prefixItems entries for tuple schemas

[Dev-X25874]

Summary

_ensure_strict_json_schema in strict_schema.py handles "items" for homogeneous
arrays but has no branch for "prefixItems" — the key Pydantic v2 emits for
fixed-length tuple types (e.g. tuple[int, MyModel]) per JSON Schema draft 2020-12.

As a result, any nested object schema inside a tuple position is never visited, so:

• "additionalProperties": false is never added to nested objects → OpenAI rejects
  the tool definition under strict mode at runtime.
• "default": null is not stripped from inner schemas.
• anyOf/oneOf inside tuple positions are not normalized.

The fix adds a parallel block immediately after the existing items block that
iterates over prefixItems entries and applies _ensure_strict_json_schema
recursively to each one, mirroring the existing handling for anyOf, oneOf,
and allOf.

Test plan

• Confirmed the bug by tracing _ensure_strict_json_schema — "prefixItems" is
  absent from the function entirely before this change.
• The existing tests/test_strict_schema.py suite covers the surrounding logic;
  a new test case for a prefixItems schema (e.g. a nested object inside a tuple
  position) should be added to confirm additionalProperties: false is propagated
  correctly.

Issue number

Checks

• I've added new tests (if relevant)
• I've added/updated the relevant documentation
• I've run make lint and make format
• I've made sure tests pass