fix: astar_search_test.cpp did not test correctness

[Becheler]

The test was simply printing.

Before submitting

• This PR targets the develop branch.
• I searched for an existing PR or issue covering the same change.
• My contribution is licensed under the Boost Software License 1.0.

Type of change

• Bug fix
• New feature or API addition
• Refactor (no behavior change)
• Documentation
• Build, CI, or tooling
• Other (specify below)

Does this PR introduce a breaking change?

• Yes (describe migration impact below)
• No

What this PR does

• removed dead code and uselss printing
• introduced BOOST_TEST
• modernized type aliases and constexpr

Motivation

Tests should not rely on printing.

Testing

Checklist

• Existing tests pass (b2 in the test/ directory).
• New behavior is covered by a test, or this is a docs / build / refactor change.
• Documentation was updated if user-facing behavior changed.
• No new compiler warnings on the platforms I built against.

[github-actions]

Boost dependency footprint vs develop (auto-generated).
PR run 31398882275 vs develop run 31385980713 (43030dbcb3).

Header-inclusion weights (graph files pulling each direct dependency in):

No header-inclusion-weight changes.

Transitive Boost modules: 68 → 68 (0)

[codecov]

Codecov Report

❌ Patch coverage is 97.36842% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 92.82%. Comparing base (bc05361) to head (43030db).
⚠️ Report is 1 commits behind head on develop.

Files with missing lines	Patch %	Lines
test/astar_search_test.cpp	97.36%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@ Coverage Diff @@## develop #558 +/- ##
===========================================
+ Coverage 88.22% 92.82% +4.60% 
===========================================
Files 392 392 Lines 27412 28176 +764 Branches 7997 7990 -7 ===========================================
+ Hits 24183 26155 +1972 - Misses 1881 1916 +35 + Partials 1348 105 -1243 

Files with missing lines	Coverage Δ
test/astar_search_test.cpp	98.79% <97.36%> (+4.15%)	⬆️

... and 288 files with indirect coverage changes

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[github-actions]

Compiler-warning counts vs develop (auto-generated).
PR run 31398882144 vs develop run 31385980794 (43030dbcb3).

Job	Baseline	After	Delta
macos (clang, 14)	681	680	-1
macos (clang, 17)	641	641	0
macos (clang, 20)	641	641	0
ubuntu (clang-19, 14)	681	680	-1
ubuntu (clang-19, 17)	641	641	0
ubuntu (clang-19, 20)	641	641	0
ubuntu (clang-19, 23)	641	641	0
ubuntu (gcc-14, 14)	827	827	0
ubuntu (gcc-14, 17)	949	949	0
ubuntu (gcc-14, 20)	949	949	0
ubuntu (gcc-14, 23)	949	949	0
windows_msvc_14_3 (msvc-14.3)	962	959	-3