fix: remove stale Shipyard Neo core config connectivity warning

[zouyonghe]

What to build

Remove or update the core dashboard config-save Shipyard Neo connectivity warning now that concrete sandbox runtimes have been extracted into sandbox provider plugins. Core config save should not inspect removed provider-specific keys such as shipyard_neo_endpoint or shipyard_neo_access_token.

Acceptance criteria

• Saving core config with sandbox booter set to a provider id does not read removed Shipyard Neo core config keys.
• Provider-specific connectivity validation, if still needed, lives with the provider/plugin configuration path.
• Existing tests that assert the old warning are removed or replaced with provider/plugin-scoped coverage.
• Config save messaging no longer shows misleading Bay/Shipyard Neo warnings for plugin-managed providers.

Blocked by

None - can start immediately

[zouyonghe]

Handled in PR #8093 on branch feat/generic-sandbox-core.