feat(MultiSelect): hide select all when search yields no results

[mrholek]

Summary

Adds the hideSelectAllOnSearchNoResults option to the Multi Select component (default true).

When search is enabled and the query matches no options, leaving the select all button above the empty "No results found" message is misleading — there is nothing left to select. By default the component now hides the dropdown header in that case and restores it as soon as the search matches at least one option again.

Set hideSelectAllOnSearchNoResults: false to keep the header (and the select all button) visible even with no results. The option only affects the built-in select all button — a custom headerTemplate is always rendered.

Changes

• js/src/multi-select.js: new hideSelectAllOnSearchNoResults config option, header reference tracking, and _updateSelectAllVisibility() wired into the search filter.
• docs/content/forms/multi-select.md: new "Hiding select all when search has no results" subsection (with two interactive examples) and an API table entry, marked added-in 5.27.0.
• Unit tests covering the default hide behaviour and the false opt-out.

Testing

• karma unit suite: 2971 passing.

[coveralls]

Coverage Report for CI Build 29293626956

Coverage increased (+0.008%) to 94.636%

Details

• Coverage increased (+0.008%) from the base build.
• Patch coverage: 8 of 8 lines across 1 file are fully covered (100%).
• No coverage regressions found.

Uncovered Changes

No uncovered changes found.

Coverage Regressions

No coverage regressions found.

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Coverage Stats

Relevant Lines:	7463
Covered Lines:	7187
Line Coverage:	96.3%
Relevant Branches:	3088
Covered Branches:	2798
Branch Coverage:	90.61%
Branches in Coverage %:	Yes
Coverage Strength:	771.04 hits per line

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💛 - Coveralls