[Repo Assist] Add docs and tests for nonNullable, hideObjectMethods, AddCustomAttribute and ProvidedMeasureBuilder

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Addresses maintainer requests in #170 (docs + tests) and #67 (docs).

Summary

technical-notes.md additions

Two new sections were added at the end of the technical notes:

Type Definition Properties: nonNullable, hideObjectMethods, and Custom Attributes (addresses #170)

• nonNullable = true — how it adds AllowNullLiteralAttribute; how to read back NonNullable
• hideObjectMethods = true — what it suppresses in IntelliSense
• AddCustomAttribute — how to attach custom attribute data to provided types, methods, properties, and parameters; with inline CustomAttributeData examples; clarification that ProvidedConstructor does not support AddCustomAttribute (use AddObsoleteAttribute instead)

Units of Measure with ProvidedMeasureBuilder (addresses #67)

• Standard SI units via ProvidedMeasureBuilder.SI (symbols and names)
• Annotating numeric types: ProvidedMeasureBuilder.AnnotateType
• Compound units: Product, Ratio, Square, Inverse, One
• Non-standard (custom) unit type abbreviations using erased ProvidedTypeDefinition
• Multi-argument AnnotateType for generic types like Vector(float, kg)

BasicErasedProvisionTests.fs additions (10 new tests)

Tests for issue #170 topics:

• nonNullable=true sets NonNullable and adds AllowNullLiteralAttribute
• nonNullable=false (default) does not add AllowNullLiteralAttribute
• hideObjectMethods=true sets HideObjectMethods
• hideObjectMethods=false (default) leaves HideObjectMethods false
• AddCustomAttribute on a method is visible
• AddCustomAttribute on a parameter is visible (already covered in existing check custom attributes test)

Tests for issue #67 topics:

• ProvidedMeasureBuilder.SI symbol creates a FSharpTypeAbbreviation
• ProvidedMeasureBuilder.AnnotateType produces IsFSharpUnitAnnotated
• Compound unit combinators (Product, Ratio, Square, Inverse, One) are all non-null and composable
• SI name (lowercase) also creates a FSharpTypeAbbreviation

Test Status

All tests pass locally:

Passed! - Failed: 0, Passed: 94, Skipped: 1, Total: 95

(1 skipped test is lambdas - failing, an existing infrastructure skip unrelated to these changes)

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