fix(fast-build): render ?attr boolean bindings correctly in Rust renderer

[janechu]

Pull Request

📖 Description

Before: <input ?disabled="" data-fe-c-0-1>
After (truthy): <input disabled data-fe-c-0-1>
After (falsy): <input data-fe-c-0-1>

Root cause: resolve_attribute_bindings_in_tag in attribute.rs was treating all {{expr}} attribute values as string replacements. It had no awareness of the ? prefix convention used by FAST to mark boolean attributes.

Fix: Before replacing a {{expr}} binding, the function now checks whether the accumulated output ends with ?name=" — indicating a boolean binding. If so, it:

1. Strips the ?name=" prefix from the output
2. Evaluates the expression as a boolean using the existing expression::evaluate
3. Emits the bare attribute name if truthy, or nothing if falsy

📑 Test Plan

Six new tests were added to tests/hydration.rs covering:

• ?attr="{{expr}}" with a truthy value → bare attribute rendered
• ?attr="{{expr}}" with a falsy value → attribute omitted
• Comparison expression: ?disabled="{{a == b}}" (both matching and non-matching)

All 127 Rust tests pass (cargo test in crates/microsoft-fast-build).

✅ 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 documentation and followed the standards for this project.

⏭ Next Steps

The WASM artifacts in packages/fast-build/wasm/ need to be rebuilt from this crate fix (npm run build:wasm -w @microsoft/fast-build) before the fix is available to JavaScript consumers.