Delegate organization on wikis and snippets so org maintenance mode covers their LFS/Git access

What does this MR do and why?

Fixes two container types that slipped through the organization maintenance mode guard in Gitlab::GitAccess#check_organization_maintenance! (introduced in !251067 (merged)).

The guard checks container.respond_to?(:organization) before enforcing read-only mode during org maintenance. Two container types were not covered:

1. Wikis (ProjectWiki / GroupWiki): Wiki defined no organization method, so respond_to?(:organization) returned false and the guard was skipped entirely. Fix: add delegate :organization, to: :container in Wiki — both Project and Group (via Namespace) already have belongs_to :organization, so the delegation works for both wiki types.

2. Project snippets: ProjectSnippet responds to organization (via belongs_to :organization in Snippet), but organization_id stays NULL by design — a validated CHECK constraint (check_82c1d40fab) enforces exactly one of organization_id/project_id on the snippets table, and the org is resolved via project instead. So container.organization returned nil and the guard no-oped. An initial fix overriding organization on Snippet to fall back to project&.organization was rejected: Snippet also runs validates_with ExactlyOnePresentValidator, fields: [:project, :organization], and the override made both fields present for a ProjectSnippet, failing that check and breaking all 21 examples in spec/models/snippet_spec.rb. The shipped fix instead overrides check_organization_maintenance! in Gitlab::GitAccessSnippet to check snippet.organization || project&.organization, colocated with the existing #project override there, leaving the container-agnostic base class in Gitlab::GitAccess unchanged.

References

• Closes #621577 (closed)
• Follow-up to !251067 (comment 3712950024)

Screenshots or screen recordings

N/A — backend-only change.

How to set up and validate locally

1. In a rails console, create an organization and put it in maintenance, then create a project in that org.
2. Attempt a git push to the project wiki — it should now be blocked with the maintenance mode error.
3. Create a project snippet in that project and attempt a git push — it should also be blocked.

MR acceptance checklist

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