feat: Adds the ability to create hierarchical (nested) stages in Chat Completion responses.

[korotaav48]

Applicable issues

• fixes Support sub-stages (hierarchical stages) in Chat Completion responses #444

Description of changes

• Choice.create_stage() — added optional keyword argument parent: Stage | None to specify a parent stage
• Stage — exposed a public stage_index property to retrieve the stage's index (needed to pass into parent=)
• StartStageChunk — added parent_stage_index: int | None field; only included in the wire format when set (not None)
• Covered by tests in tests/test_stage_parent_index.py

Usage example:
parent = choice.create_stage("Planning")
child = choice.create_stage("Step 1", parent=parent)

Checklist

• Title of the pull request follows Conventional Commits specification

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