[26.2] Fix registry-based conditions in datapack registry and reloadable registry loading - #3451

This PR fixes registry-based loading conditions (neoforge:tag_empty, neoforge:registered) in datapack registry and reloadable registry loading.

Every "layer" of datapack loading (datapack registry, reloadable registry, reload listener) is given access to the contents and tags of all prior layers and static registries. To make this work for tag contents, the condition context must interogate both the "context registries" and the captured "pending tags". This is due to a discrepancy in tag loading between initial load and reload:

• On initial load only static registry tags are in the "pending tags" while datapack registry tags are bound immediately
• On reload both static and datapack registry tags are in the "pending tags"

This is however only an issue for introspecting tag contents during loading, the HolderSets for all tags are accessible via the "context registries", they may just not be bound yet.

The test already tries testing for access to tags on reloadable registries but 26.2 does not load tags for reloadable tags, so the non-empty case is excluded and marked as TODO for 26.3 where reloadable registries use the same loading code as datapack registries and therefore have working tags.

I also took the liberty to guard loading condition evaluation, forwarding the error cleanly to the caller instead of letting the exception bubble up into code that is at best capable of handling the GSON exceptions, if any at all.

At the moment, this PR does not pass formatting because I had to use a patch for an access change due to a limitation in JST.

Fixes #3441