fix(niagara): validate create/duplicate/template destinations before source load

[kunkunGames]

Goal

Reject malformed Niagara create, duplicate, and template-save destinations before loading source assets, invoking AssetTools, or calling CreatePackage.

Plain-English summary

The destination should be validated before looking up a source asset or template. Invalid input now returns a normal tool error without loading the source, creating an empty package, or entering the duplicate/save flow.

Improvements

• Validates seven owning entry points: system, stateless emitter, HLSL script, system duplicate, NPC, effect type, and emitter template.
• Validates template-based create_system destinations before template lookup and load.
• Reuses the create_system owner guard from create_system_from_spec instead of adding a duplicate validation layer.
• Covers the current stateless-emitter CreatePackage path.
• Uses FPackageName helpers consistently after validation.
• Exercises a real valid Niagara create/save/delete lifecycle with an isolated package name.

Before → After

Before	After
Some paths only checked for a slash before package or AssetTools calls	Every owning destination must be a valid writable Unreal long package name
Duplicate and template-save paths could load the source before rejecting the destination	Destination errors return before source or template load
The stateless-emitter package path was outside the earlier guard set	All current direct Niagara CreatePackage owners are protected
The valid regression reused a fixed package and removed its file directly	Each run uses a GUID package and deletes it through the Editor Asset API after file notifications settle

Side-effect analysis

• Malformed destination: loads no source/template asset and creates no package.
• Template-based create: performs template lookup and duplication only after destination validation succeeds.
• create_system_from_spec: parses the spec and delegates to the validated owner; it has no separate package owner.
• Valid /Game/... destination: preserves existing factory initialization, Asset Registry, dirtying, and save behavior.
• Empty or missing required fields retain their existing dedicated errors.
• The new EditorScriptingUtilities dependency is private and is used only by the automation fixture cleanup.

Implementation

• Applied the shared package-path validator to seven Niagara owning handlers.
• Connected direct package creation, AssetTools duplication, and template-save destinations to validated paths.
• Added a malformed scenario matrix plus a valid system-creation regression.
• Replaced fixed-path, direct-file cleanup with a GUID fixture and delayed Editor Asset API deletion.
• Declared the cleanup API's owning module explicitly and synchronized the Niagara specification and changelog.

Shared backlog bookkeeping (read before merging out of order)

Docs/specs/SPEC_MonolithCore.md tracked this work as ~24 of 80 CreatePackage call sites guarded; remaining ~56 ... follow-up backlog, while MonolithPackagePathValidator.h separately claimed ~77 sites remain unguarded. Both numbers were already inconsistent on master and go stale every time an owner is wired.

This PR replaces both with a routed-owner list plus a named remaining backlog, and points the header at the spec so there is a single place to update.

That shared line is updated only in this PR, not in #106 or #107. All three would otherwise edit the same single line and conflict on merge. The list therefore names #106, #107, and #108 as routed owners — if any of those is dropped or deferred, say so and I will trim the line in one commit.

Verification

• UE 5.8 MonolithPRHostEditor Win64 Development build: passed (11 actions after dependency invalidation).
• Monolith.Niagara.PackagePath.*: 2/2 passed.
  • Malformed matrix covers seven owners, template create, and spec delegation (9 scenarios).
  • Valid Niagara system creation, save, delayed Editor Asset API deletion, and post-notification absence passed.
• No test-window warnings or errors.
• Generated NS_ValidPackagePath_*.uasset residue count: 0.
• git diff --check: passed.

[kunkunGames]

Conflict/merge-order note for my six open PRs is in #104 (comment #104 (comment)).

Short version: a pairwise trial merge shows these PRs conflict only in CHANGELOG.md (plus Docs/specs/SPEC_MonolithCore.md for the #104 × #112 pair) — no source conflicts. Merge in any order that suits your batch; I will rebase the remainder and re-verify with a build plus automation tests.

[tumourlove]

Shipped in v0.21.3 — thanks. Seven Niagara entry points validate the destination before loading a source asset, touching AssetTools, or creating a package, and the hand-rolled FindLastChar('/') splitting is replaced with FPackageName::GetLongPackagePath / GetLongPackageAssetName. Confirmed create_system_from_spec delegates to the guarded HandleCreateSystem rather than needing its own check, as you said.

Two changes from the branch:

I dropped the EditorScriptingUtilities addition to MonolithNiagara.Build.cs. It was only needed by the valid-path test fixture, and I did not take those fixtures — WITH_DEV_AUTOMATION_TESTS is 1 in Development editor builds, which is what ships, so those tests register in end users' Session Frontend and write a real NiagaraSystem into their project with cleanup behind two latent waits. An aborted run leaves residue. The malformed-path cases create nothing, so those are the ones worth having, and I put the real coverage on the validator itself where it is a pure function.

On the shared backlog line: you were right that it was inconsistent on master and right to flag the merge-order coupling. I went further and deleted the count rather than fixing it — bookkeeping that three separate changes have to edit in lockstep is bookkeeping that ends up wrong. It is now a pointer to grep ValidatePackagePath for current owners.

Thanks for the ordering note in the description, incidentally — it is the reason I did not end up with three conflicting edits to one line.

I land contributor fixes as my own commits rather than merging the branch — shipped history stays single-author, reporter credited in the release notes.

https://github.com/tumourlove/monolith/releases/tag/v0.21.3