[DREAM-693] Unify and document ExpandableTextComponent [FOLLOW UP - CSS vars]

[myabc]

Note

This PR is based off #23328. Please merge first.

Ticket

https://community.openproject.org/wp/DREAM-693 (follow up)

What are you trying to accomplish?

Drive ExpandableText line clamp via CSS variable

Replaces the per-count op-vertical-truncate--lines-N modifier classes with an inline --op-vertical-truncate-lines custom property, set through Primer's join_style_arguments like Primer::Beta::Truncate does for max-width. This drops the generated-class explosion and the hard upper bound, so any count from 2 up works. The floor is 2 because a single clamped line is single_line's job, not multi_line's.

Screenshots

no visual changes

What approach did you choose and why?

see #23328 (comment)

Merge checklist

• Added/updated tests
• Added/updated documentation in Lookbook (patterns, previews, etc)
• Tested major browsers (Chrome, Firefox, Edge, ...)

[Copilot]

Pull request overview

This follow-up to DREAM-693 changes the vertical (multi-line) truncation implementation to drive -webkit-line-clamp via an inline CSS custom property, avoiding per-line-count modifier classes and enabling arbitrary line counts (with a minimum of 2). It updates component specs and Lookbook documentation/previews accordingly.

Changes:

• Replace .op-vertical-truncate--lines-N usage with an inline --op-vertical-truncate-lines style set via join_style_arguments.
• Update vertical truncate styling to use line-clamp(var(--op-vertical-truncate-lines, 3)) and remove the generated modifier-class loop.
• Adjust RSpec / frontend Stimulus controller specs and Lookbook docs/previews to assert the new CSS-variable behavior.

Reviewed changes

Copilot reviewed 9 out of 9 changed files in this pull request and generated 2 comments.

Show a summary per file

File	Description
app/components/op_primer/vertical_truncate_component.rb	Applies line count via inline --op-vertical-truncate-lines and clamps only to a minimum of 2.
app/components/op_primer/vertical_truncate_component.sass	Switches line-clamp styling from modifier classes to a CSS variable-based clamp.
app/components/op_primer/expandable_text_component.rb	Updates parameter docs to reflect “minimum 2” behavior for multi-line mode.
spec/components/op_primer/vertical_truncate_component_spec.rb	Updates expectations to assert the inline CSS variable instead of modifier classes.
spec/components/op_primer/expandable_text_component_spec.rb	Updates multi-line truncation assertions to the CSS-variable approach.
spec/components/op_primer/expandable_text_component_preview_spec.rb	Updates preview expectations to match the CSS-variable approach.
lookbook/previews/op_primer/expandable_text_component_preview.rb	Expands Lookbook control range for lines and keeps previews aligned with variable-based truncation.
lookbook/docs/components/expandable-text.md.erb	Updates documentation to describe “2 or more” and CSS variable-driven clamp.
frontend/src/stimulus/controllers/expandable-text.controller.spec.ts	Updates fixtures to use --op-vertical-truncate-lines instead of --lines-N classes.