Add support for data values schema without defaults

**Describe the problem/challenge you have**

Some data values don't have sensible defaults and I'd like template users to always provide them

**Describe the solution you'd like**

Maybe special value for `@schema/default` annotation , which would indicate that there is no deafult value at all? Schema is then used to define just type of the value, expecting template user to always provide it.

**Anything else you would like to add:**

Currently it can be simulated with assert, but it is too verbose.

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Start by tracing how the `@schema/default` annotation and schema-defined data values are handled, using the existing `assert` workaround as a behavioral reference. Define how a schema marks a value as having no default, then verify that template users must provide it while its type remains enforced.

Written by the indexing model from the issue text.