Set explicit uri and base on NonRelativeURI type (#545)

Fixes shaclgen "No URI for type" and sqltablegen "Unknown range base"
errors. Workaround for LinkML generators not traversing `typeof`.

Workaround for linkml/linkml#3491.

`make all` produced a clean build but two streams of generator errors.
Both stem from the same upstream LinkML behaviour: `gen-shacl` and
`gen-sqltable` do not walk `typeof` to resolve type attributes. The fix
is to redeclare on `NonRelativeURI` what it would otherwise inherit from
its parent `uri` type.

## Changes

Two attributes added to the `NonRelativeURI` type definition in
`src/sssom_schema/schema/sssom_schema.yaml`:

```yaml
NonRelativeURI:
  typeof: uri
  base: URI            # added
  uri: xsd:anyURI      # added
  description: ...
```

**1. `uri: xsd:anyURI` — silences `gen-shacl`**

Without this, `shaclgen` emits:

```
ERROR:linkml.generators.shaclgen:No URI for type NonRelativeURI
```

once per slot whose range is `NonRelativeURI` (15 occurrences). The
SHACL generator reads the type's `uri` directly to populate
`sh:datatype` and does not fall back to the parent type. `xsd:anyURI` is
the URI declared on the LinkML built-in `uri` type — semantically a
`NonRelativeURI` *is* an `xsd:anyURI` with an extra (prose-only)
restriction, so this is the correct value rather than a minted
`sssom:NonRelativeURI` (which no SHACL processor would understand).

**2. `base: URI` — silences `gen-sqltable`**

Without this, `sqltablegen` emits:

```
ERROR:linkml.generators.sqltablegen:Unknown range base: None for mapping_set_id = NonRelativeURI
ERROR:linkml.generators.sqltablegen:Unknown range base: None for license = NonRelativeURI
... (one error per slot referencing NonRelativeURI)
```

The SQL generator uses `base` to pick a SQL column type and, like
shaclgen, doesn't traverse `typeof`. `URI` matches the parent type's
`base`.

## This is a workaround

The real bug is upstream: `typeof` is documented as inheritance, but
`SchemaView.get_type()` returns child types as-declared and individual
generators reimplement (or skip) the walk. Other generators in the same
build (`gen-python`, `gen-jsonld`, `gen-jsonschema`, `gen-graphql`,
`gen-shex`) handle the same schema fine, which makes the inconsistency a
generator-side problem rather than a schema-author responsibility. Filed
upstream — see linkml/linkml#3491.

Until that lands, redeclaring inherited attributes on every custom type
is the established LinkML convention (cf. `EntityReference` in the same
file, which sets `base: str` and `uri: rdfs:Resource` for the same
reason).

## Verification

`make all -B` after the change: no `shaclgen` or `sqltablegen` errors.
Remaining output is GraphQL warnings about enum values like `1:1`, `n:n`
and imported class names like `owl class` (other problem)

- [ ] ~~`docs/` have been added/updated if necessary~~
- [X] `make test` has been run locally
- [ ] ~~tests have been added/updated (if applicable)~~
- [ ]
~~[CHANGELOG.md](https://github.com/mapping-commons/sssom/blob/master/CHANGELOG.md)
has been updated.~~

If you are proposing a change to the SSSOM metadata model, you must 

- [ ] provide a full, working and valid example in `examples/`
- [ ] provide a link to the related GitHub issue in the `see_also` field
of the linkml model
- [ ] provide a link to a valid example in the `see_also` field of the
linkml model
- [ ] update the "Model changes across versions" (in
`src/docs/spec-models.md`) accordingly
- [ ] run SSSOM-Py test suite against the updated model

Author: matentzn

src/sssom_schema/schema/sssom_schema.yaml

@@ -131,6 +131,8 @@ types:
       - https://mapping-commons.github.io/sssom/spec/#tsv
   NonRelativeURI:
     typeof: uri
+    base: URI
+    uri: xsd:anyURI
     description: >-
       A URI as per RFC 3986, that is a string that matches the production of the
       "URI" rule defined in Appendix A of that RFC. Contrary to the underlying