getattributes(::DimArray): always a Dict with String keys

[bjarthur]

The axis interface says getattributes returns a Dict keyed by Strings. For a DimArray the DimensionalData extension returned metadata(x) as is, which is NoMetadata() for an array built without metadata, or a Metadata wrapper, or a Dict keyed by Symbols.

Downstream, YAXArrays stores that object as the YAXArray's properties, and savedataset then fails with MethodError: no method matching copy(::NoMetadata). Users work around it by constructing the DimArray with metadata = Dict{String,Any}() (see JuliaDataCubes/YAXArrays.jl#362 and its MWE).

This normalises the three cases to Dict{String,Any}: NoMetadata to an empty Dict, a Metadata wrapper to its Dict, and any AbstractDict to one with String keys. Anything else is passed through unchanged.

julia> using YAXArrayBase, DimensionalData
julia> getattributes(DimArray(rand(2, 3), (X(1:2), Y(1:3))))
# before: NoMetadata()          after: Dict{String, Any}()
julia> getattributes(DimArray(rand(2, 3), (X(1:2), Y(1:3)); metadata = Dict(:units => "K")))
# before: Dict(:units => "K")   after: Dict{String, Any}("units" => "K")

Tests added to the DimensionalData test item for all three cases.

🤖 Generated with Claude Code

[codecov]

Codecov Report

❌ Patch coverage is 80.00000% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 52.22%. Comparing base (49d7155) to head (7e9c6c0).
⚠️ Report is 2 commits behind head on master.

Files with missing lines	Patch %	Lines
ext/DimensionalDataExt.jl	80.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master      #70      +/-   ##
==========================================
+ Coverage   52.04%   52.22%   +0.18%     
==========================================
  Files          12       12              
  Lines         490      494       +4     
==========================================
+ Hits          255      258       +3     
- Misses        235      236       +1     

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