[SaladDay]

A Session whose frozen MCP Credential binding names a Credential that was deleted (or whose Vault was deleted) now rejects new message input with the same 400 selection error that Session creation returns for an absent Credential, instead of admitting a Turn that fails at dispatch.

• vaults.Service.CheckMCPCredentials reads metadata only, reusing FindMCPCredentials.
• Worker.SubmitInputs runs the check for message batches on a Session with no active Turn, before and outside the admission transaction.
• A retry of a batch the Session already admitted still replays (sessions.Service.InputAdmitted).
• The ledger line for this difference is removed; vaults.md and the sessions-events.md input error table describe the behavior (EN + ZH).

Checks: focused go test for vaults, execution, sessions, api and the new integration test mcp_credential_input_test.go.

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