fix: resolve nullable type lookup bypassing custom value parsers

[natemcmaster]

Fix nullable type lookup in ValueParserProvider.GetParserImpl<T>() to check user-registered parsers under the unwrapped type before falling back to TypeDescriptor.

This fixes the bug where AddOrReplace with a custom parser for a built-in type like TimeSpan was ignored when the property type was TimeSpan?.

Fixes #559

Generated with Claude Code

[claude]

Claude finished @natemcmaster's task in 1m 7s —— View job

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Code Review: PR #608

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Thanks for the fix — this is clean and correct.

The bug: In GetParserImpl<T>(), the _defaultValueParserFactory.TryGetParser<T>() fallback (which uses TypeDescriptor) was checked before the nullable-unwrap block that looks up _parsers[wrappedType]. So for TimeSpan?, the default factory would return a built-in parser for the nullable type, and the user-registered parser for TimeSpan was never consulted.

The fix: Moving the default factory fallback after the nullable unwrap ensures the lookup order is:

1. _parsers[TimeSpan?] (exact match)
2. Unwrap nullable → _parsers[TimeSpan] (user-registered for inner type) → wrap in NullableValueParser
3. _defaultValueParserFactory (built-in fallback)

This correctly prioritizes user-registered parsers over the built-in defaults for nullable types. For non-nullable types, the nullable block is skipped entirely, so behavior is unchanged.

Test coverage is solid — the three new tests cover custom parser with nullable, custom parser with non-nullable, and default behavior without a custom parser.

No issues found. ✅

[natemcmaster]

@claude update this PR with release notes on the fix

[claude]

Claude finished @natemcmaster's task in 40s —— View job

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Updated PR with release notes

• Read current release notes files
• Update releasenotes.props and CHANGELOG.md
• Commit and push changes

Added release notes for the nullable value parser fix (#559) to both:

• CHANGELOG.md — under the Unreleased Fixes section
• src/CommandLineUtils/releasenotes.props — under the 5.1.x Fixes section