fix(helpers): resolve .msh format ambiguity between Ansys and GMSH writers

[fznoussi]

fix(helpers): resolve .msh format ambiguity between Ansys and GMSH writers

When converting a MED file to MSH format:

$ meshio convert mesh.med output.msh

the output was silently written by the Ansys writer instead of the GMSH
writer, producing a file with no metadata and no error:

expected: GMSH .msh with $PhysicalNames, $Entities, $Nodes

actual: Ansys .msh with no groups, no physical names

Root cause: the .msh extension is shared between Ansys and GMSH.
_filetypes_from_path() returns both formats and the code was blindly
picking the first one:

file_formats = _filetypes_from_path(Path("output.msh"))

-> ['ansys', 'gmsh']

file_format = file_formats[0]

-> 'ansys' <- Ansys writer called silently

Fix A : Add _pick_best_format() to resolve ambiguous extensions

When multiple formats share the same extension, the mesh metadata
is inspected to determine the most appropriate writer.

GMSH is preferred over Ansys when any of the following keys are
present in cell_data, point_data or field_data:

• GMSH-specific keys : gmsh:physical, gmsh:geometrical, gmsh:dim_tags
• MED-specific keys : cell_tags, point_tags, med:*

def _pick_best_format(file_formats, mesh):
if "gmsh" in file_formats:
gmsh_keys = {"gmsh:physical", "gmsh:geometrical", "gmsh:dim_tags"}
med_keys = {"cell_tags", "point_tags"}
has_gmsh = bool(gmsh_keys & set(mesh.cell_data.keys()))
or bool(gmsh_keys & set(mesh.point_data.keys()))
has_med = bool(med_keys & set(mesh.cell_data.keys()))
or bool(med_keys & set(mesh.point_data.keys()))
or any(k.startswith("med:") for k in mesh.field_data)
if has_gmsh or has_med:
return "gmsh"
return file_formats[0]

The fallback to file_formats[0] preserves the original behaviour for
all cases where no metadata hint is available.

Fix B : Call _pick_best_format() only when ambiguity exists

_pick_best_format() is only invoked when more than one format matches
the extension, avoiding any overhead on unambiguous extensions:

BEFORE

file_format = file_formats[0]

AFTER

if len(file_formats) > 1:
file_format = _pick_best_format(file_formats, mesh)
else:
file_format = file_formats[0]

Confirmed against mesh.med generated by GMSH 4.13:

$ meshio convert mesh.med output.msh
$ meshio info output.msh

Before: Ansys writer selected, 1 block, no $PhysicalNames
After : GMSH writer selected, 6 blocks, $PhysicalNames contains "subgroup"

Fixes nschloe#1541