fix: add null checks and return code validation for KECCAK256

[Copilot]

Addresses review feedback on KECCAK256 implementation to add defensive validation and proper error propagation.

Changes

• Null pointer validation: jet_ops_keccak256 checks ctx for null before dereferencing, returns -1 on null
• Return code propagation: Caller checks jet_ops_keccak256 return code and branches to ReturnCode::Invalid on error, following call_mem_expand_checked pattern
• Minimal unsafe scope: Consolidated unsafe operations into small blocks with SAFETY comments per Rust 2024 edition requirements

// Before: return code ignored
bctx.builder.build_call(
    bctx.env.symbols().keccak256(),
    &[ctx.into(), offset.into(), size.into(), result.into()],
    "keccak256",
)?;

// After: check and branch on error
let ret = bctx.builder.build_call(...)?;
let ret_i8 = unsafe { IntValue::new(ret.as_value_ref()) };
let is_ok = bctx.builder.build_int_compare(EQ, ret_i8, zero, "keccak256_ok")?;
bctx.builder.build_conditional_branch(is_ok, ok_block, err_block)?;

Safety documentation clarified: ctx must be valid if non-null (runtime check handles null case), result must always be valid and non-null (UB if null).

---

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[semanticdiff-com]

Review changes with  

Changed Files

File	Status
crates/jet_runtime/src/builtins.rs	31% smaller
crates/jet/src/builder/ops.rs	1% smaller

[Copilot]

Pull request overview

This PR adds defensive validation and proper error propagation to the KECCAK256 implementation, ensuring robust error handling for potential null context pointers and invalid memory ranges. The changes align the KECCAK256 builtin with existing patterns used for memory expansion and other operations that interact with the execution context.

Changes:

• Added null pointer check in jet_ops_keccak256 before dereferencing the context pointer, returning -1 on null
• Implemented return code checking in the caller with explicit branching to ReturnCode::Invalid on error, following the call_mem_expand_checked pattern
• Consolidated unsafe operations into minimal scopes with comprehensive SAFETY comments per Rust 2024 edition requirements

Reviewed changes

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

File	Description
crates/jet_runtime/src/builtins.rs	Added null pointer validation and memory range checks to jet_ops_keccak256, with minimal unsafe blocks and clear SAFETY documentation
crates/jet/src/builder/ops.rs	Added return code checking and error branching logic to keccak256 opcode implementation, mirroring the call_mem_expand_checked pattern