feat: DH-21476: Middleware plugin infrastructure for widget chaining

[vbabich]

Middleware Plugin Infrastructure for Widget Chaining

Summary

Adds middleware plugin support so plugins can wrap existing widgets without replacing them, cross-plugin dependency loading via manifest package/dependencies fields, and shared plugin-loading utilities in @deephaven/plugin.

What Changed

Middleware chaining — New WidgetMiddlewarePlugin type, identified by its own PluginType.MIDDLEWARE_PLUGIN discriminator. Middleware receives a Component prop and wraps the next layer. Multiple middleware compose in registration order. Applied in both WidgetLoaderPlugin (dashboard panels) and WidgetView (inline widgets) via createChainedComponent/createChainedPanelComponent. The chaining functions automatically filter middleware by supportedTypes at render time, so middleware only activates for matching widget types.

Cross-plugin dependencies — Manifest entries can declare package (makes the plugin's exports available to other plugins) and dependencies (topological sort so deps load first). Plugins load sequentially so each plugin's exports are available to subsequent plugins via standard import statements.

Shared loading utilities — getPluginModuleValue, registerPlugin, processLoadedModule, sortPluginsByDependency, and manifest types moved from @deephaven/app-utils into @deephaven/plugin. Both web-client-ui and gplus consume these instead of duplicating the logic. ~80 lines removed from app-utils.

Behavior change — duplicate plugin names. When two plugins in the manifest are loaded under the same name, the shared registerPlugin now skips the duplicate and keeps the first registration (with a warning). Previously the duplicate replaced the existing entry. This applies to plugin-name collisions in the plugin map; widget-type resolution in WidgetLoaderPlugin is unaffected (last base plugin still wins for a given widget type).

Key Files

File	Change
packages/plugin/src/PluginTypes.ts	Middleware types, type guard
packages/plugin/src/PluginUtils.tsx	Chaining functions, loader utils, manifest types
packages/plugin/src/sortPluginsByDependency.ts	Topological sort for plugin dependencies
packages/plugin/src/WidgetView.tsx	Middleware-aware inline rendering
packages/dashboard-core-plugins/src/WidgetLoaderPlugin.tsx	Middleware-aware panel loading
packages/app-utils/src/plugins/PluginUtils.tsx	Simplified to use shared utils
packages/plugin/docs/middleware-architecture.md	Architecture reference

BREAKING CHANGE: PluginManifestPluginInfo, PluginManifest, and
getPluginModuleValue have been moved from @deephaven/app-utils to
@deephaven/plugin. Update imports from @deephaven/app-utils to
@deephaven/plugin.

[codecov]

Codecov Report

❌ Patch coverage is 88.01843% with 26 lines in your changes missing coverage. Please review.
✅ Project coverage is 50.05%. Comparing base (3ed691d) to head (5b3e45f).
⚠️ Report is 3 commits behind head on main.

Files with missing lines	Patch %	Lines
packages/plugin/src/WidgetView.tsx	0.00%	23 Missing ⚠️
.../dashboard-core-plugins/src/WidgetLoaderPlugin.tsx	96.29%	2 Missing ⚠️
packages/plugin/src/PluginUtils.tsx	99.21%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2660      +/-   ##
==========================================
+ Coverage   49.54%   50.05%   +0.50%     
==========================================
  Files         774      774              
  Lines       43898    44074     +176     
  Branches    11308    11171     -137     
==========================================
+ Hits        21749    22060     +311     
+ Misses      22103    21996     -107     
+ Partials       46       18      -28     

Flag	Coverage Δ
unit	50.05% <88.01%> (+0.50%)	⬆️

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[mofojed]

Looks good overall, just a couple comments/questions

[vbabich]

Our e2e tests fail in main on deephaven:edge.
deephaven:latest works fine; the issue must be introduced on the deephaven-core side.

[mofojed]

Looks good, will merge after next ui release as this has breaking changes.

[mofojed]

Deprecate instead of break

[mofojed]

Instead of making this a breaking change and removing these, mark them deprecated and re-export the types from @deephaven/plugin.