fix: Fix modifier types

[alanbsmith]

Summary

Fixes: #3895
Update type of createStencil to allow modifier merging when extending stencils.

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Components

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• MDX documentation adheres to Canvas Kit's Documentation Guidelines
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[cypress]

Workday/canvas-kit    Run #11544

Run Properties:   Passed #11544  •  b3987ebc5e ℹ️: Merge 35462e4c9f0cfaac4a1d1c95d2550ad6bfe1b5c4 into 1beac223ba75b47655ae919851bd...

Project	Workday/canvas-kit
Branch Review	ISSUE-3895
Run status	Passed #11544
Run duration	02m 36s
Commit	b3987ebc5e ℹ️: Merge 35462e4c9f0cfaac4a1d1c95d2550ad6bfe1b5c4 into 1beac223ba75b47655ae919851bd...
Committer	Alan B Smith
View all properties for this run ↗︎

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Test results
Failures	0
Flaky	0
Pending	17
Skipped	0
Passing	824
View all changes introduced in this branch ↗︎

UI Coverage  19.41%
Untested elements	1563
Tested elements	374

Accessibility  99.44%
Failed rules	5 critical   5 serious   0 moderate   2 minor
Failed elements	68

[Copilot]

Pull request overview

This PR updates the TypeScript typing for createStencil so modifier types merge correctly when a stencil extends another stencil, addressing the modifier inference issue reported in #3895.

Changes:

• Introduce RemoveIndexSignature/MergeModifiers to prevent index-signature-driven inference issues when combining modifier configs.
• Apply MergeModifiers in the relevant Stencil* conditional types so extended + local modifiers compose cleanly.
• Update unit tests to no longer require modifiers: {} on a base stencil for boolean modifier scenarios.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
modules/styling/lib/cs.ts	Fixes extended-stencil modifier type composition by merging modifier types while stripping index signatures that break inference.
modules/styling/spec/cs.spec.tsx	Removes now-unnecessary modifiers: {} setup in tests, validating improved type inference/usage ergonomics.

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• modules/styling/lib/cs.ts
• modules/styling/spec/cs.spec.tsx

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[Copilot]

Pull request overview

Copilot reviewed 2 out of 2 changed files in this pull request and generated no new comments.