fix: parse filter elements(...) syntax in f-children directive

[Prasad48432]

Pull Request

📖 Description

This PR fixes parsing of the filter elements(...) syntax for the declarative f-children directive.

Previously, TemplateParser passed the entire directive value directly to children(), causing expressions such as f-children="{childItems filter elements(li)}" to be treated as a literal property name instead of parsing the filter. As a result, no element filter was applied.

This change extracts the filter parsing into a shared helper used by both the children and slotted directive cases. When a filter elements(...) clause is present, the parser now constructs the appropriate options object and passes it to children(), while preserving the existing behavior for directives without filters.

🎫 Issues

Closes #7631

👩‍💻 Reviewer Notes

• The change is isolated to packages/fast-element/src/declarative/template-parser.ts.
• Parsing of the filter elements(...) syntax is now shared between the children and slotted directive implementations to avoid duplicate logic.
• Existing behavior for f-children="{property}" is preserved. The options object is only used when a filter is specified.

📑 Test Plan

Added declarative parser tests covering:

• f-children="{listItems}" resolves to a ChildrenDirective with the expected property and no filter.
• f-children="{childItems filter elements()}" produces a filter that accepts element nodes and rejects text nodes.
• f-children="{childItems filter elements(li)}" produces a selector-based filter that accepts <li> elements and rejects other elements and text nodes.

✅ Checklist

General

• I have included a change request file using $ npm run change
• I have added tests for my changes.
• I have tested my changes.
• I have updated the project documentation to reflect my changes.
• I have read the [CONTRIBUTING](https://github.com/microsoft/fast/blob/main/CONTRIBUTING.md) documentation and followed the [standards](https://github.com/microsoft/fast/blob/main/CODE_OF_CONDUCT.md#our-standards) for this project.

Agents

• I have linked to an existing issue in this project that this change addresses.
• [x ] I have read the skills.
• [x ] I have read the DESIGN.md file(s) in packages relevent to my changes.
• I have updated the DESIGN.md file(s) in packages relevent to my changes.

⏭ Next Steps

• Add a change file using npm run change if required by the maintainers.
• Update documentation if the declarative f-children filter syntax is intended to be part of the public API.

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