test(graduation): give the width tests' tag ownership of its mesh

[gouarin]

Description

make_graduation_width_test_tag, added in #498, built the mesh as a local variable and
returned a tag on it. A field only points to its mesh (inner_mesh_type stores a raw pointer),
so graduation.width_zero_is_a_noop and graduation.width_above_dispatch_range_throws run
samurai::graduation on a destroyed mesh.

On main the two tests pass by luck: the stack slot is still intact when the tag is read. On
the branches rebased on main today (#492, #493, #494) the full test run fails with

Expected: samurai::graduation(tag, stencil) doesn't throw an exception.
  Actual: it throws std::out_of_range with description
  "LevelCellArray::get_interval: interval not found at level 4098, i = [0,8[@0:2, index = ".

while the same test passes when run alone. AddressSanitizer confirms the cause on main
itself:

ERROR: AddressSanitizer: stack-use-after-return
    #0 samurai::graduation<...>  include/samurai/algorithm/graduation.hpp:141
    #1 graduation_width_zero_is_a_noop_Test::TestBody()  tests/test_graduation.cpp:284
Address is located in stack of thread T0 at offset ... in frame
    #0 make_graduation_width_test_tag  tests/test_graduation.cpp:267

The tag is now built on holder(mesh): the field owns a copy of the mesh, so the helper is safe
by construction, the way reconstruction() already returns a field that outlives its local mesh.

Returning the mesh and building the tag in each test was considered and rejected: it keeps the
lifetime rule at every call site, and a one-liner passing a temporary mesh would recreate the bug
without any warning.

How has this been tested?

• tests/test_graduation.cpp, built with -fsanitize=address: the report above before the
  change, clean after it, graduation.* all passing.
• pre-commit run --files tests/test_graduation.cpp.

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[codacy-production]

Up to standards ✅

🟢 Issues 0 issues

Results:
0 new issues

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🟢 Metrics 0 complexity · 0 duplication

Metric	Results
Complexity	0
Duplication	0

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