fix(ocs): [OCISDEV-1435] announce vault storage provider on every capabilities response

[gauravsoni119]

vault_storage_provider was only filled in on capabilities?vault=true, so clients outside the vault got vault.enabled: true with an empty provider id. Their vault checks never matched: a vault permanent link opened in a normal tab failed to resolve, vault notifications leaked into the Drive view, and the invite form didn't recognize vault resources.

The id is a fixed constant, not a secret, so it is now set once in Init whenever vault mode is enabled and appears in every capabilities response. The ?vault=true branch also no longer mutates the handler's shared capabilities struct, which made the old behaviour intermittent.

Tests: new unit tests in capabilities_test.go (fail on the old code, pass with the fix).

Jira: OCISDEV-1435

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[gauravsoni119]

Closing this. We're fixing it on the web side instead: web now uses the vault storage provider id as a constant, like the share jail and OCM ids, so no reva change is needed. See owncloud/ocis#13001. Thanks for the suggestion!