[6.x] Keep augmentable tag results as objects inside Antlers interpolations

[duncanmcclean]

This pull request fixes an issue where assigning the asset tag's result to a variable, like {{ img = { asset url="/img/logo.png" } }}, gave you the URL string rather than the asset, so {{ img.alt }}, {{ img.width }} and friends were empty.

This was happening because tag output inside an interpolation was reduced as if it were being rendered directly, and that path casts any object with __toString (like an Asset) to a string before the value is handed back.

This PR fixes it by keeping augmentable objects intact only when the interpolation is the right-hand side of an assignment. Interpolations used anywhere else, like dynamic keys ({{ items.{tag} }}), condition operands and expressions, are reduced to strings exactly as before. The Antlers runtime already knows how to work with an Asset held in a variable, so {{ img }} still outputs the URL, while {{ img.alt }}, {{ img:width }} and looping over {{ img }} now work too.

Since img now holds the asset rather than a string, {{ if img === "/img/logo.png" }} is false, the same as it is for an asset from a field. Loose == still matches.

Fixes #11257

[jasonvarga]

Thanks for digging into this one — the diagnosis is right, and the fix does resolve #11257. {{ img = { asset url="..." } }} keeps the Asset now, and the paired form starts working as a bonus.

The problem is scope. The guard keys off $this->isInterpolationProcessor, which is shared by three consumers, and two of them require a scalar. That leaves two regressions, one of which is a hard error:

template	6.x (e76bf9c)	this branch
{{ img = { asset } }}{{ img }}|{{ img.alt }}	/assets/a.jpg|	/assets/a.jpg|Alpha ✅ fixed
{{ img = { asset } }}{{ img }}[{{ alt }}]{{ /img }}	(empty)	[Alpha] ✅ improved
{{ if {asset} == "/assets/a.jpg" }}	yes	yes
{{ if {asset} === "/assets/a.jpg" }}	yes	no ⚠️
{{ items[{asset}] }}	matched	empty ⚠️
{{ items.{asset} }}	matched	TypeError ❌

Measured on PHP 8.5 against a tag returning an Asset whose URL is /assets/a.jpg. CI is green because no test covers either of the broken paths.

Suggested direction: keep the object only for the assignment consumer, and coerce Stringable results to string at the key-consuming boundaries in PathDataManager, rather than gating inside NodeProcessor::reduce() where all three consumers share the flag. Details inline.