fix: make VsagException public inherit from std::exception

[inabao]

Summary

Cherry-pick of PR #1776 to branch 0.17

• Fix VsagException to use public inheritance from std::exception instead of private inheritance
• Reorder catch blocks in engine.cpp to ensure VsagException is caught before std::exception
• This allows VsagException to be caught by catch (std::exception& e) handlers, following standard C++ exception handling patterns

Problem

The class was using private inheritance (default for class):

class VsagException : std::exception {  // private inheritance

This prevented catching VsagException with std::exception handler, breaking standard exception handling.

Solution

Changed to public inheritance:

class VsagException : public std::exception {

And reordered catch blocks to ensure derived class (VsagException) is caught before base class (std::exception).

Changes

1. src/vsag_exception.h: Changed to public inheritance
2. src/factory/engine.cpp: Reordered catch blocks (VsagException before std::exception)

Impact

• No breaking changes - all existing code behavior unchanged
• Adds ability to catch VsagException via std::exception handler
• Follows C++ standard practice for exception classes

[gemini-code-assist]

Code Review

This pull request updates the VsagException class to use public inheritance from std::exception and reorders the catch blocks in Engine::CreateIndex to ensure that the more specific VsagException is caught before the general std::exception. I have no feedback to provide.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

@@            Coverage Diff             @@
##             0.17    #1787      +/-   ##
==========================================
- Coverage   91.90%   91.46%   -0.44%     
==========================================
  Files         318      318              
  Lines       17758    17758              
==========================================
- Hits        16320    16243      -77     
- Misses       1438     1515      +77     

Flag	Coverage Δ
cpp	91.46% <100.00%> (-0.44%)	⬇️

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Components	Coverage Δ
common	91.75% <ø> (ø)
datacell	93.20% <ø> (-0.04%)	⬇️
index	90.62% <ø> (-0.30%)	⬇️
simd	100.00% <ø> (ø)

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[LHT129]

LGTM