fix(microsoft-fast-build): normalize attribute keys and strip client-only bindings from SSR output

[janechu]

Summary

Fixes how the microsoft-fast-build Rust SSR crate handles custom element attributes when building child rendering state and producing declarative shadow DOM HTML. Rebased on top of #7398 (data-* → dataset mapping).

Attribute → child state key rules

Binding form	Child state key	Rationale
isEnabled="..."	isenabled	HTML attrs are case-insensitive; browsers lowercase
selected-user-id="42"	selected-user-id	Hyphens preserved
foo="{{bar}}"	foo (resolved from parent state)	Standard double-brace binding
data-date-of-birth="..."	{"dataset": {"dateOfBirth": ...}}	Handled by #7398
:myProp="{{expr}}"	(skipped)	Client-side only — FAST runtime sets JS properties
@click="{fn()}"	(skipped)	Client-side only

Attribute value rules

• Boolean attributes (no value, e.g. disabled) → true in child state
• All string attributes → stored as strings — count="42" is "42", not the number 42
• Typed values must be passed via {{binding}} expressions so the resolved JsonValue flows from parent state

Client-only bindings — stripped from HTML, skipped from state

Both @event and :property bindings are removed from all rendered HTML (outer element open tag and all tags inside the declarative shadow DOM) and are not added to the child element's rendering scope. The data-fe-c binding count is preserved so the FAST runtime still allocates the correct number of binding slots.

Changes

• directive.rs: @ and : prefixed attrs both skipped when building child state; all HTML attribute keys lowercased; attribute_to_json_value simplified (strings only, no numeric/boolean coercion for literals); dataset logic from feat(microsoft-fast-build): map data-* attributes to nested dataset state #7398 preserved
• attribute.rs: strip_client_only_attrs removes @attr and :attr from any opening tag; refactored to use parse_element_attributes + read_tag_name
• node.rs: strip_client_only_attrs wired into hydration tag scan so shadow template HTML is also cleaned
• Tests updated; test_custom_element_colon_property_binding_skipped verifies property bindings are stripped from HTML and not passed to child scope; stale test names fixed
• DESIGN.md and README.md updated to document the full attribute handling rules alongside the dataset mapping from feat(microsoft-fast-build): map data-* attributes to nested dataset state #7398

[Copilot]

Pull request overview

This PR updates @microsoft/fast-build’s server-side renderer so custom element state-building better matches FAST runtime conventions (camelCase property access, : property bindings, and skipping @ event bindings), and also adds support for accessing .length on arrays in template expressions.

Changes:

• Build child element state with :-prefix stripping, kebab-case → camelCase aliasing, and skipping @event attributes.
• Add .length support when resolving nested properties on arrays.
• Expand test coverage for the above, plus JSON array/object literals in custom element attributes.

Reviewed changes

Copilot reviewed 9 out of 9 changed files in this pull request and generated 5 comments.

Show a summary per file

File	Description
crates/microsoft-fast-build/src/directive.rs	Adjusts custom element state construction (@ skip, : strip, kebab→camel alias) and extends attribute value parsing (JSON literals).
crates/microsoft-fast-build/src/context.rs	Adds array .length handling in nested property resolution.
crates/microsoft-fast-build/tests/f_when.rs	Adds f-when condition tests using items.length.
crates/microsoft-fast-build/tests/custom_elements.rs	Adds tests for kebab→camel aliasing, : property bindings, @ skipping, and JSON literal attribute parsing.
crates/microsoft-fast-build/tests/bindings.rs	Adds direct binding tests for items.length (including nested and empty cases).
crates/microsoft-fast-build/README.md	Documents new custom-element attribute/state behaviors (:/@ handling, kebab→camel alias).
crates/microsoft-fast-build/DESIGN.md	Updates design notes for state-building rules (skip @, strip :; camel alias).
change/@microsoft-fast-build-fix-kebab-bindings-1314e0de-ff4c-4dd5-8cc5-2e8fc04bc0aa.json	Patch change file for kebab→camel and : binding fixes.
change/@microsoft-fast-build-fix-array-length-3d13d21c-1594-4fe1-babf-8123aa5e098e.json	Patch change file for array .length support.