Clarify Session workspace ownership

[SaladDay]

Hosted Environments are Session-scoped under the pinned public contract. Clarify this boundary in the English and Chinese coverage ledger and verify that a request carrying an existing Environment reference is rejected before template lookup or storage writes, rather than silently creating a new workspace. No production behavior or public schema changes.

Validation: complete Core API package tests and vet passed; a mutation check confirmed the new handler test detects accepting the unsupported field. Translation checks (3), documentation checks (6), and website link build passed. Coordinator reviewed the complete diff; no new blind review for this small documentation/test change under the user’s review scope decision.

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