[#71351] Teach Single/Range Date Picker support for placeholder text

[myabc]

Important

This PR is based off #21723. Please review/merge that PR first.

Ticket

https://community.openproject.org/work_packages/71351

What are you trying to accomplish?

• Updates Angular OpBasicSingleDatePickerComponent and OpBasicRangeDatePickerComponents to support passing a placeholder value.
• Updates Primer Form inputs to pass placeholder as a date attribute.
• Additionally, this PR makes SingleDatePickerInput's datepicker_options param optional.

Screenshots

Before	After

Merge checklist

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

[Copilot]

Pull request overview

Adds placeholder support to the Primer form DatePicker components by plumbing the placeholder value from the Ruby form DSL through the Rails-rendered Angular component tag into the Angular datepicker inputs.

Changes:

• Make datepicker_options optional for SingleDatePickerInput (and by inheritance RangeDatePickerInput), simplifying call sites and specs.
• Pass placeholder through DatePicker’s Rails template into Angular component inputs.
• Add Angular @Input() placeholder bindings and new specs validating data-placeholder output for single and range date pickers.

Reviewed changes

Copilot reviewed 9 out of 9 changed files in this pull request and generated 1 comment.

Show a summary per file

File	Description
spec/lib/primer/open_project/forms/dsl/input_methods_spec.rb	Updates DSL specs to reflect datepicker_options now being optional.
spec/lib/primer/open_project/forms/date_picker_spec.rb	Adds coverage asserting placeholder is emitted as data-placeholder for both date picker variants.
modules/backlogs/app/forms/backlogs/backlog_header_form.rb	Removes redundant datepicker_options: {} now that the default exists.
lib/primer/open_project/forms/dsl/single_date_picker_input.rb	Makes datepicker_options default to {} to support optional usage.
lib/primer/open_project/forms/date_picker.html.erb	Wires placeholder into Angular component inputs (currently always emitted).
frontend/src/app/shared/components/datepicker/basic-single-date-picker/basic-single-date-picker.component.ts	Adds @Input() placeholder.
frontend/src/app/shared/components/datepicker/basic-single-date-picker/basic-single-date-picker.component.html	Binds placeholder to the rendered <input> elements.
frontend/src/app/shared/components/datepicker/basic-range-date-picker/basic-range-date-picker.component.ts	Adds @Input() placeholder.
frontend/src/app/shared/components/datepicker/basic-range-date-picker/basic-range-date-picker.component.html	Binds placeholder to the rendered <input> elements.