fix: preserve service-specific plugin mounts

[Pigbibi]

Summary

• Preserve the exact existing per-service plugin mount document when the central switch uses plugin_mode=current.
• Read nested env mounts first, fail closed on ambiguity or conflicting source values, and use the historical repository-level lookup only when no service target is found.
• Add unit tests, including a nested per-service TQQQ mount.

Validation

• PYTHONPATH=python python -m unittest python.tests.test_runtime_settings
• PYTHONPATH=python python python/scripts/build_config.py --check
• PYTHONPATH=python python python/scripts/build_platform_config.py --check

This change neither enables a target nor changes an order; it prevents an incumbent plugin from being erased during a continuity transition.