Add organization-aware primary key lookups and statement cache

What does this MR do and why?

Adds organization-aware safety to Rails find-by-primary-key calls and to the ActiveRecord statement cache.

Two pieces land together because the statement cache patch is required for the PK lookup functions to be used safely under connection multiplexing:

1. PostgreSQL lookup functions — find_<table>_by_id_and_organization_id for projects, namespaces, and users. Each function does a WHERE id = $1 AND organization_id = $2 fetch, ensuring a find can never return a record belonging to a different organization.

2. OrganizationAwareStatementCache — patches the ActiveRecord statement cache key to include the current organization ID. Without this, a cached prepared statement bound to one organization could be reused for another.

Factories and specs for models that cross organization boundaries are updated to supply an explicit organization: association where the new constraint requires it.

References

• Replaces: Application-Level Database Isolation (!225550 - closed)
• Relates to Isolate read queries on Organization sharded mo... (#582272)

How to set up and validate locally

• Run the migrations in this MR: bundle exec rake db:migrate
• Start a Rails console and execute the following statements:

# This works, project id 1 belaongs to organization 1
Project.find(1)

# enable Feature flag
Feature.enable(:data_isolation)

# Now we set Current Organization to another organization and mark it as an isolated org 
Current.organization = Organizations::Organization.find_or_create_by!(path: 'my-isolated-org', name: 'My Isolated Org')
Current.organization.mark_as_isolated!

# This will now raise an error because project id 1 does not exists in this new organization
Project.find(1)

MR acceptance checklist

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