fix: DH-23165: Fix spectrum Picker closing Popper dialogs on open

[vbabich]

Add optional containPortals prop to the Popper dialog to allow nested spectrum Pickers and not dismiss the dialog on Picker open.

Verified manually on the http://localhost:4220/ide/styleguide#popover-dialog page and the PivotBuilder plugin aggregations config section (will post a separate PR in the deephaven-plugins repo).

Added unit and e2e tests.

[Copilot]

Pull request overview

Adds opt-in portal containment to Popper, preventing nested Spectrum overlays from dismissing their parent dialog.

Changes:

• Introduces containPortals using React Aria’s portal provider.
• Adds stacking-context isolation for contained overlays.
• Adds styleguide, unit, and E2E regression coverage.

Reviewed changes

Copilot reviewed 6 out of 13 changed files in this pull request and generated no comments.

Show a summary per file

File	Description
packages/components/src/popper/Popper.tsx	Implements portal containment.
packages/components/src/popper/Popper.scss	Isolates contained overlay stacking.
packages/components/src/popper/Popper.test.tsx	Tests opt-in and default rendering.
packages/components/package.json	Adds the React Aria dependency.
packages/code-studio/src/styleguide/Dialog.tsx	Adds a nested Picker example.
tests/styleguide.spec.ts	Adds the E2E visual regression test.
package-lock.json	Records the dependency update.

[codecov]

Codecov Report

❌ Patch coverage is 44.44444% with 5 lines in your changes missing coverage. Please review.
✅ Project coverage is 51.51%. Comparing base (b59031c) to head (a77c9e8).

Files with missing lines	Patch %	Lines
packages/code-studio/src/styleguide/Dialog.tsx	0.00%	5 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2726      +/-   ##
==========================================
+ Coverage   51.40%   51.51%   +0.10%     
==========================================
  Files         797      797              
  Lines       45852    45859       +7     
  Branches    11906    11720     -186     
==========================================
+ Hits        23569    23622      +53     
+ Misses      22264    22218      -46     
  Partials       19       19              

Flag	Coverage Δ
unit	51.51% <44.44%> (+0.10%)	⬆️

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