feat(sdk): add Step.children

[namrataghadi-galileo]

Summary

• Add optional recursive children to the provider-neutral runtime Step model.
• Pass supplied children through evaluate_controls() so trace and session evaluators can inspect the complete tree.

Scope

• User-facing/API changes: evaluate_controls() accepts children: list[Step] | None. Child steps retain their structured fields and can contain nested children.
• Internal changes: children are attached to the current step and serialized recursively when present. Omitted or None children are excluded from serialization to preserve legacy payloads.
• Out of scope: child steps are not evaluated recursively; @control behavior and trace/session lifecycle handling are unchanged. No provider-specific fields or logic were added.

Risk and Rollout

• Risk level: low. The new field is optional and requests without children keep their prior serialized shape.
• Rollback plan: remove the optional model field and SDK argument; existing callers and payloads require no migration.

Testing

• Added or updated automated tests for recursive validation, serialization compatibility, forwarding, single-step evaluation, and unchanged local matching.
• Focused SDK tests: 65 passed.
• Model tests: 71 passed.
• Ruff and targeted mypy passed.
• make check was not run: dependency resolution against the configured package index failed with HTTP 403.
• Full SDK suite: 685 passed, 14 skipped, 1 failed. The failure was test_client_context_manager, which requires a server at http://localhost:8000 that was not running.

Checklist

• Linked issue/spec (if applicable)
• Updated docs/examples for user-facing changes: no docs/examples change was needed for this additive SDK argument.
• Included any required follow-up tasks

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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