feat(api): add shared variable contracts

[tkkhq]

Tracking

• Tracks: https://konghq.atlassian.net/browse/VOL-985
• Related: https://github.com/Kong/volcano-hosting/pull/1007

Why

• SDK users need the function-variable scope and shared-membership contract exposed by Hosting
• conditional shared-list replacement must avoid sending the complete membership twice

Summary

• add shared variable metadata, function scope fields, and complete-list replacement
• add mutually exclusive list and digest preconditions
• constrain digests to 64 lowercase hexadecimal characters
• regenerate the public Python API client

Verification

• uv run python scripts/generate_openapi.py
• uv run pytest tests
• uv run ruff check .
• uv run ruff format --check .
• uv run mypy