feat: validate strategy release target identity

[Pigbibi]

Summary

• accept an optional but complete immutable strategy release identity in RUNTIME_TARGET_JSON
• reject partial, malformed, or unexpected release fields
• retain compatibility for legacy targets during the read-only migration

Verification

• jq empty schemas/runtime-target.schema.json
• uv run --project python python -m unittest discover -s python/tests -p test_runtime_settings.py

This PR does not add a release identity to any target and does not deploy runtime settings.

[Pigbibi]

验证说明：本 PR 的本地受限测试已通过（117 tests）。GitHub Python CI 的失败发生在严格跨项目依赖矩阵检查，报告的是多个其他仓库既有的 QPK/策略 pin 漂移；本 PR 只改 runtime target schema、校验器和对应测试，未更新这些依赖。为避免把无关的大范围 pin 修复混入发布身份契约，本 PR 保持未合入，待矩阵单独恢复一致后再合入。