Add name-based unit abbreviation lookup overloads

[angularsen]

Motivation

#1067 asks for looking up unit abbreviations by quantity and unit names instead of requiring unit enum values. The parser side and UnitInfo overloads already exist, but the direct UnitAbbreviationsCache.Default.GetDefaultAbbreviation("Length", "Millimeter") shape was still missing.

Closes #1067.

Changes

• Added UnitAbbreviationsCache.GetDefaultAbbreviation(string quantityName, string unitName, IFormatProvider? formatProvider = null).
• Added UnitAbbreviationsCache.GetUnitAbbreviations(string quantityName, string unitName, IFormatProvider? formatProvider = null) for the same lookup shape.
• Added tests for valid lookups, case-insensitive names, configured quantity lookups, and invalid/null inputs.

Validation

• dotnet test UnitsNet.Tests --filter "FullyQualifiedName~UnitAbbreviationsCacheTests"
• dotnet test UnitsNet.Tests --no-build --filter "FullyQualifiedName~UnitAbbreviationsCacheTests"

[claude]

Review

Small, focused addition — new name-based overloads on UnitAbbreviationsCache, no generated-code or unit-definition changes, so most of the checklist below doesn't apply.

Breaking changes: None. Purely additive public API (two new overloads with an optional trailing parameter), no changes to generated code or existing signatures.

New quantities/units: N/A — no changes to Common/UnitDefinitions/.

Generated code: N/A — only hand-written code in UnitsNet/CustomCode/UnitAbbreviationsCache.cs and tests.

Code quality / conventions:

• The new GetDefaultAbbreviation(string, string, IFormatProvider?) / GetUnitAbbreviations(string, string, IFormatProvider?) overloads mirror the existing shape used elsewhere (e.g. QuantityInfoLookup.GetUnitByName, Quantity.From(string, string, ...)), so the API is consistent with existing name-based lookup patterns in the library.
• GetConfiguredUnitInfo(string quantityName, string unitName) (UnitsNet/CustomCode/UnitAbbreviationsCache.cs:521-527) correctly delegates to the instance's Quantities (not UnitsNetSetup.Default.Quantities), so it respects a cache constructed with a restricted quantity set — confirmed by the QuantityAndUnitNameOverloads_UseConfiguredQuantityLookup test.
• Null checks use the if (x is null) throw new ArgumentNullException(nameof(x)) style already used in this file (e.g. line 517), consistent with the rest of the file rather than introducing ArgumentNullException.ThrowIfNull or another convention.
• XML doc comments are thorough and document all thrown exception types (ArgumentNullException, QuantityNotFoundException, UnitNotFoundException), matching the style of neighboring members.

Potential bugs: None spotted. The null checks happen before the lookup, and case-insensitivity is delegated to QuantityInfoLookup.GetUnitByName, which already does an OrdinalIgnoreCase comparison — the new "case insensitive" test documents existing lookup behavior rather than new logic, which is fine.

Test coverage: Good — covers success, case-insensitivity, restricted-quantity-set behavior, invalid quantity/unit names, and null argument names for both new overloads.

Performance/security: No concerns — this is a thin convenience wrapper around existing lookup logic.

Nice, minimal PR overall.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 96%. Comparing base (9cd9c6d) to head (dce28df).

Additional details and impacted files

@@           Coverage Diff           @@
##           master   #1695    +/-   ##
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  Coverage      96%     96%            
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  Files         457     455     -2     
  Lines       30163   29558   -605     
=======================================
- Hits        29098   28519   -579     
+ Misses       1065    1039    -26     

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