fix(grid): preserve Excel filter density with custom heights

[simeonoff]

Closes #17589

Description

Fix Excel-style filter menu density when minHeight or maxHeight is set.

• Move sizable() to the .igx-excel-filter base class so sizing flags remain available regardless of custom height constraints.
• Let the menu inherit --component-size from the host instead of redeclaring it.
• Keep density-dependent default height limits in __sizing.
• Add regression tests and update the Grid README.

Motivation / Context

Setting either height input removes the __sizing class, which previously also removed the sizing flags. Without a sizing ancestor—for example, in an external outlet—the menu fell back to large padding and gaps even when --ig-size selected small or medium density.

Initializing sizing on the host separates density from height constraints and preserves the existing custom-height behavior.

Type of Change (check all that apply):

• Bug fix
• New functionality
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Refactoring (no functional changes)
• Documentation
• Demos
• CI/CD
• Tests
• Changelog
• Skills/Agents

Component(s) / Area(s) Affected:

• Grid Excel-style filtering (IgxGridExcelStyleFilteringComponent)
• Shared Excel-style filter structural styles and density inheritance
• Grid filtering UI tests and README

How Has This Been Tested?

• Unit tests
• Manual testing
• Automated e2e tests

Added regression coverage for:

• Small, medium, and large densities in an external outlet.
• No custom bounds, minHeight only, maxHeight only, and both bounds.
• Setting and clearing bounds while the menu is open, including restoration of default heights.
• Runtime density changes on a standalone filter outside the grid.
• Computed menu header/main padding and main-section gaps.

Test Configuration:

• Angular version: 22.1.1 (installed test dependency)
• Browser(s): Chrome Headless 154.0.0.0
• OS: macOS 27.0.1

Checklist:

• All relevant tags have been applied to this PR
• This PR includes unit tests covering all the new code (test guidelines)
• This PR includes API docs for newly added methods/properties (api docs guidelines) — N/A: no new public API.
• This PR includes feature/README.MD updates for the feature docs
• This PR includes general feature table updates in the root README.MD — N/A: no new feature.
• This PR includes CHANGELOG.MD updates for newly added functionality — N/A: bug fix; released-version entries are unchanged.
• This PR contains breaking changes
• This PR includes ng update migrations for the breaking changes (migrations guidelines) — N/A: no breaking changes.
• This PR includes behavioral changes and the feature specification has been updated with them — Restores expected density behavior; no specification update.
• Accessibility (ARIA, keyboard navigation, focus management) has been verified — No accessibility behavior was changed; no separate accessibility audit was performed.

[Copilot]

Copilot review overview

🟢 Approval recommended

The targeted style fix is consistent with the component structure and has comprehensive regression coverage.

Review effort: Balanced
Findings: None

What changed in this PR

Fixes Excel-style filter density when custom height constraints are used.

Changes:

• Moves sizing flags to the filter host.
• Adds density and height regression coverage.
• Documents sizing behavior.

File	Description
grid-samples.spec.ts	Adds the external-outlet test fixture.
grid-filtering-ui.spec.ts	Tests densities, bounds, and runtime updates.
README.md	Documents filter sizing behavior.
_base.scss	Preserves sizing variables independently of height constraints.

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