Add unit tests for WasmEngine and SandboxPolicy (engine.rs)

[niklabh]

Description

oxide-browser/src/engine.rs defines WasmEngine and SandboxPolicy — the core compilation and sandboxing layer. It currently has no tests.

This module is critical for security (fuel limits, memory bounds), so test coverage would catch regressions early.

Suggested test cases

• SandboxPolicy defaults — verify SandboxPolicy::default() returns expected fuel (500M) and memory page limits (256)
• Engine creation — WasmEngine::new() succeeds with a valid policy
• Module compilation — compile a minimal valid .wasm module (can use wat::parse_str to create one inline)
• Fuel limit enforcement — a module with an infinite loop should trap after fuel is exhausted
• Memory limit enforcement — a module that tries to grow memory beyond the limit should fail
• Invalid WASM — compiling garbage bytes should return an error, not panic

Files involved

• oxide-browser/src/engine.rs — add a #[cfg(test)] mod tests { ... } block

Helpful references

• wasmtime testing guide
• WAT (WebAssembly Text Format) can be used to create inline test modules

Difficulty

Intermediate — requires basic understanding of wasmtime and WebAssembly concepts.