Return error result when unit labels aren't matched during quantity parsing

[hl662]

This pull request improves error handling in the @itwin/core-quantity package's quantity parser. The parser now returns an explicit error result when a unit label is supplied but cannot be matched to any known unit, rather than silently failing or accepting invalid labels. This is a breaking change that ensures more accurate feedback for consumers of the parsing API.

Error handling improvements:

• The Parser class now throws a QuantityError with QuantityStatus.UnitLabelSuppliedButNotMatched when unique unit labels are present but not matched to any known unit.
• The main parsing logic catches this error and returns a result with ParseError.UnitLabelSuppliedButNotMatched, ensuring callers receive a clear error when parsing fails due to unknown unit labels.

Testing and documentation:

• Added a new test case to Parsing.test.ts to verify that parsing with invalid unit labels returns the expected error.
• Updated the change history documentation to highlight this, warning that parsing operations with invalid unit labels will now return errors instead of succeeding silently.

[hl662]

I will highlight that none of our previous tests failed, there are no regressions so far, including this relevant test case where when a unitless format is passed in, the parser will use the first unit label that matches any known units for parsing

const testData = [
  { value: "12,345.345 - 1", magnitude: 12345.345 - 1}, // unitless
  { value: "1m + 1FT + 1", magnitude: 1 + 0.3048 + 1}, // default to meters
  { value: "1FT + 1m + 1", magnitude: 0.3048 + 1 + 0.3048}, // default to feet
  { value: "1 + 1 FT + 1m", magnitude: 0.3048 + 0.3048 + 1 }, // default to feet
];

[Copilot]

Pull Request Overview

This PR improves error handling in the quantity parser by returning explicit error results when unit labels cannot be matched to known units, replacing the previous silent failure behavior.

• Enhanced the Parser.getDefaultUnitConversion method to throw a QuantityError when unit labels are present but unmatched
• Updated the main parsing logic to catch these errors and return ParseError.UnitLabelSuppliedButNotMatched results
• Added comprehensive test coverage for invalid unit label scenarios

Reviewed Changes

Copilot reviewed 4 out of 4 changed files in this pull request and generated 2 comments.

File	Description
core/quantity/src/Parser.ts	Added error throwing for unmatched unit labels and error handling in parsing logic
core/quantity/src/test/Parsing.test.ts	Added test cases to verify proper error handling for invalid unit labels
docs/changehistory/NextVersion.md	Documented the breaking change for enhanced error handling
common/changes/@itwin/core-quantity/nam-parser-label-fix_2025-09-30-14-41.json	Added change log entry for the parser enhancement

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