feat: DH-22891: Example middleware plugin

[vbabich]

Adds table-middleware-example, a minimal WidgetMiddlewarePlugin demonstrating how a middleware plugin is inserted into a widget's render chain and how it can wrap the next component and inject custom props - with no coupling to IrisGrid or any other specific widget.

What it does

• A single shared body hook (exampleMiddlewareBody) returning { inject, wrap }: injects a custom exampleInjectedProp and wraps the
  wrapped component in an ExampleMiddlewareContext.Provider plus a small
  "Wrapped by example middleware" banner.
• The one body hook drives both middleware paths via the
  @deephaven/plugin factories: createWidgetMiddleware (non-panel /
  dashboard) and createPanelMiddleware (panel path, which preserves
  golden-layout's panel ref so componentState persistence stays intact).
• Wired up as a PluginType.MIDDLEWARE_PLUGIN with supportedTypes: ['Table', 'TreeTable', 'HierarchicalTable', 'PartitionedTable'].

The middleware example needs to be registered in plugins/manifest.json to be loaded in dev mode.

[Copilot]

Pull request overview

This PR adds a new example plugin, table-middleware-example, under plugins/table-middleware-example/src/js. It demonstrates the WidgetMiddlewarePlugin mechanism: a middleware component is inserted into a widget's render chain (for Table, TreeTable, HierarchicalTable, and PartitionedTable) and can inject custom props onto the next component and wrap it (here, with a context provider and a small banner). A single shared body hook (exampleMiddlewareBody) drives both the non-panel widget path (createWidgetMiddleware) and the ref-preserving panel path (createPanelMiddleware). The plugin is intentionally minimal and decoupled from IrisGrid, serving as documentation-by-example for the new middleware API in @deephaven/plugin.

Changes:

• Adds a shared exampleMiddlewareBody returning { inject, wrap }, plus ExampleWidgetMiddleware/ExamplePanelMiddleware built from it via the @deephaven/plugin factories.
• Registers everything as a PluginType.MIDDLEWARE_PLUGIN (ExampleMiddlewarePlugin) with a small context (ExampleMiddlewareContext) and barrel exports.
• Adds standard plugin scaffolding (package.json, vite.config.ts, README, LICENSE, .gitignore) and a package-lock.json workspace link entry.

Reviewed changes

Copilot reviewed 11 out of 12 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
plugins/table-middleware-example/src/js/src/exampleMiddlewareBody.tsx	Shared body hook implementing inject and wrap behavior.
plugins/table-middleware-example/src/js/src/ExampleWidgetMiddleware.tsx	Non-panel widget middleware built via createWidgetMiddleware.
plugins/table-middleware-example/src/js/src/ExamplePanelMiddleware.tsx	Panel-path middleware built via createPanelMiddleware (ref-forwarding).
plugins/table-middleware-example/src/js/src/ExampleMiddlewarePlugin.ts	WidgetMiddlewarePlugin definition wiring both paths and supportedTypes.
plugins/table-middleware-example/src/js/src/ExampleMiddlewareContext.ts	Context type/provider read by descendants of the wrapped widget.
plugins/table-middleware-example/src/js/src/index.ts	Barrel exports and default export of the plugin.
plugins/table-middleware-example/src/js/package.json	Package metadata, deps, and build scripts.
plugins/table-middleware-example/src/js/vite.config.ts	Vite library build config (cjs, externalized deps).
plugins/table-middleware-example/src/js/README.md	Documentation of the example; overstates manifest registration (flagged).
plugins/table-middleware-example/src/js/LICENSE	Apache-2.0 license text.
plugins/table-middleware-example/src/js/.gitignore	Ignores node_modules and dist.
package-lock.json	Adds workspace link entry for the new plugin.

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