add simple_dict annotations to extensions

[cmungall]

Summary

Adds explicit simple_dict_value: true annotation to the extension_value slot in extensions.yaml, aligning the source schema with what's already present in derived artifacts (meta.jsonld, meta.owl.ttl).

Motivation

The extension class has three slots: extension_tag (key), extension_value (required), and extensions (recursive self-reference, not required). Generators need to know which non-key slot holds the dict value to serialize extensions as simple {tag: value} dicts.

Currently this works via heuristic inference in get_range_associated_slots() — extension_value is the only non-key required slot, so it gets picked automatically. This annotation makes that intent explicit rather than relying on the fallback heuristic.

Notes

• No functional change — generators already handle this correctly via inference
• The derived artifacts (meta.jsonld, meta.owl.ttl) already contain this annotation from a prior manual edit; this brings the source YAML into sync

EDIT: @matentzn added this PR description in the hope that it reflects @cmungall original intend.

[matentzn]

@cmungall please add some basic text to the PR to understand the motivation and possible ramifications of this change.

[matentzn]

This adds simple_dict_value: true to extension_value in extensions.yaml. The extension class has three slots: extension_tag (key), extension_value (required), and extensions (recursive, not required). The thing is, get_range_associated_slots() in helpers.py already correctly identifies extension_value as the simple dict value slot via fallback heuristic #3: it's the only non-key required slot. So generators already handle this correctly without the annotation.

The simple_dict_value annotation in the derived artifacts (meta.jsonld, meta.owl.ttl) was likely added manually at some point — there's no code in linkml that auto-generates it during schema compilation. It's a stale/orphaned artifact, not evidence that the source YAML needs updating.

So this PR is purely defensive: making implicit behavior explicit. That's fine, but it's not fixing anything broken. The real question is whether we want to treat simple_dict_value annotations as required documentation or whether the heuristic-based inference is good enough. If we're going to annotate, we should probably do it consistently across all simple dict patterns, not just extensions.

[amc-corey-cox]

This appears to me to be a simple low-risk change. I'm inclined to approve.

But it does beg the question: should we make this change everywhere that could use this as well? What makes extensions special in this case? As things stand this slot can be in-lined as a simple dict without this change based on how linkml allows simple dict in-lining and this PR makes no functional change.

[amc-corey-cox]

@cmungall Can you give a response to my and @matentzn reviews before we decide to merge this? I see nothing blocking this but I'm not sure if we actually need or want it.