fix(grids): getState no longer mutates live sorting/groupBy/filtering expressions

[viktorkombov]

Closes #17105

Additional information (check all that apply):

• Bug fix
• New functionality
• Documentation
• Demos
• CI/CD

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)
• This PR includes feature/README.MD updates for the feature docs
• This PR includes general feature table updates in the root README.MD
• This PR includes CHANGELOG.MD updates for newly added functionality
• This PR contains breaking changes
• This PR includes ng update migrations for the breaking changes (migrations guidelines)
• This PR includes behavioral changes and the feature specification has been updated with them

[Copilot]

Pull request overview

Fixes a grid state regression where calling getState() mutated the grid’s live sorting/groupBy/filtering expression objects (e.g., deleting strategy/owner), which could break later re-application of those expressions.

Changes:

• Clone sorting and grouping expressions before stripping non-serializable props (strategy/owner) so live expressions are not mutated.
• Clone filtering/advancedFiltering expression trees (recursively) while stripping owner, avoiding in-place mutation of the live tree.
• Add unit tests asserting getState() does not mutate live sorting/groupBy/filtering/advancedFiltering expressions.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
projects/igniteui-angular/src/lib/grids/state.directive.spec.ts	Adds regression tests for non-mutating getState() behavior (but currently focuses the suite).
projects/igniteui-angular/src/lib/grids/state-base.directive.ts	Updates getFeatureState implementations to return cloned state instead of mutating live expressions; adds a recursive filtering tree clone helper.

[Copilot]

Pull request overview

This PR fixes a bug in the grid state directive where calling getState() mutated the grid’s live sorting/grouping/filtering expressions (e.g., deleting strategy / owner), which could break later usage of those expressions (as reported in #17105).

Changes:

• Update getState feature extractors for sorting and groupBy to return sanitized copies (instead of mutating the live expressions).
• Update filtering and advanced filtering state extraction to clone the filtering tree while stripping owner, avoiding mutation of the live tree.
• Add unit tests asserting getState does not mutate live sorting, grouping, filtering, and advanced filtering expressions.

Reviewed changes

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

File	Description
projects/igniteui-angular/src/lib/grids/state-base.directive.ts	Avoids in-place deletion of strategy/owner by copying expressions; introduces recursive filtering-tree cloning that strips owner.
projects/igniteui-angular/src/lib/grids/state.directive.spec.ts	Adds regression tests covering non-mutation of live sorting/groupBy/filtering/advancedFiltering expressions when calling getState.