[Bug] Fix undefined behavior in INT8Quantizer::ComputeImpl due to missing return statement

[LHT129]

Description

The ComputeImpl method in INT8Quantizer class has undefined behavior when the L2SQR branch is used. The function may reach the end without a return statement, which is undefined behavior for a non-void function.

Background

During code review of the quantization module, it was discovered that the ComputeImpl method in src/quantization/int8_quantizer.cpp has a code path that could lead to undefined behavior:

• The IP and COSINE branches use if constexpr, which allows compile-time branch elimination
• The L2SQR branch uses a regular if (without constexpr), so the compiler cannot eliminate the fall-through path
• If control flow doesn't enter any branch (theoretically shouldn't happen, but the compiler can't prove it), the function ends without a return statement
• Missing return in a non-void function is undefined behavior, and the compiler may generate unpredictable code

Requirements

• Add static_assert to ensure only supported metric types are used
• Change else if to else if constexpr for L2SQR branch
• Ensure all branches use if constexpr for compile-time branch elimination
• Eliminate undefined behavior

Technical Details

File Modified: src/quantization/int8_quantizer.cpp

Changes Made:

1. Added static_assert at the beginning of ComputeImpl function to validate metric type
2. Changed else if (metric == MetricType::METRIC_TYPE_L2SQR) to else if constexpr (metric == MetricType::METRIC_TYPE_L2SQR)

Acceptance Criteria

• Code compiles without warnings
• All existing tests pass
• No undefined behavior in the function
• Compile-time branch elimination works correctly for all metric types

Related

• Original task file: agent-hive/tasks/2026-03-18-修复-int8-quantizer-computeimpl-缺少-return.md
• Reference implementation: ComputeDistImpl function in the same file (lines 156-186)