Fix min/max pushforwards in Hessian generation

[Elvand-Lie]

Summary

• return std::min and std::max pushforward results by value
• preserve exact custom-derivative matches first, then retry only eligible ValueAndPushforward<const T&, const T&> returns with value types
• cover two-argument and explicit-comparator min/max Hessians

Fixes: #1872

Testing

• Regressions/issue-1872.cpp
• FirstDerivative/BuiltinDerivatives.C
• ForwardMode/STLCustomDerivatives.C confirms reference-returning derivatives retain exact matching
• full regression suite: 18 passed, 1 CUDA unsupported
• changed-line clang-format --dry-run --Werror
• git diff --check

Performance

Exact-HEAD versus patched, 11 alternating Release compile runs after warm-up:

• STLCustomDerivatives.C: 1343.6 ms -> 1390.1 ms (+3.46%, overlapping ranges)
• BuiltinDerivatives.C: 1307.6 ms -> 1271.4 ms (-2.77%)

At -O3, all four Hessian paths inline without ValueAndPushforward call or object overhead.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.

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[github-actions]

clang-tidy made some suggestions

[github-actions]

warning: avoid do-while loops [cppcoreguidelines-avoid-do-while]

    do {
    ^

[vgvassilev]

This seems to be just a single iteration why the do-while-stmt?

[Elvand-Lie]

I think the do-while(false) wasn't intended to loop I intentionally used it as a breakable scope.

Because we have to fall through to the diagnostic code at the bottom on failure, using break allowed me to keep the early exit checks perfectly flat without deep nesting.

But after I noticed the code styling guide from Github actions I've now simplified the type-checking logic down to just three conditions, I've replaced it with a standard nested if in the latest push.

[github-actions]

clang-tidy review says "All clean, LGTM! 👍"