[Repo Assist] Add DateOnly/TimeOnly inference support (closes #1461)

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Closes #1461

Summary

Adds DateOnly and TimeOnly support to FSharp.Data's type inference pipeline, as requested by @dsyme in issue #1461.

Changes

Multi-targeting

All library projects now target netstandard2.0;net8.0 (previously netstandard2.0 only). This is required for #if NET6_0_OR_GREATER guards to activate in library code.

Automatic DateOnly inference

• Date-only strings like "2022-06-11" are now inferred as DateOnly on .NET 6+ targets.
• DateOnly is a subtype of DateTime in the type hierarchy — columns mixing DateOnly and DateTime values unify to DateTime (no breaking change for mixed columns).
• AsDateOnly guards against the DateOnly.TryParse gotcha: DateOnly.TryParse("2022-06-12T01:02:03") returns true (ignores the time component). The fix checks for non-zero TimeOfDay after a successful parse and returns None in that case.

TimeOnly — explicit annotation only

TimeOnly is not automatically inferred from string values. The string "00:30:00" is valid both as TimeOnly and TimeSpan, making automatic inference ambiguous and a breaking change for existing code.

• TimeOnly is available via explicit column annotation (timeonly? or timeonly option in CSV column headers / XML/JSON schema).
• All TimeOnly runtime methods (AsTimeOnly, ConvertTimeOnly, etc.) are present.
• AsTimeOnly guards against the same gotcha as AsDateOnly: TimeOnly.TryParse("2016-10-05T04:05:03") returns true.

Files changed

File	Change
**/FSharp.Data*.fsproj (5 files)	netstandard2.0 → netstandard2.0;net8.0
TextConversions.fs	AsDateOnly, AsTimeOnly with datetime-string guards
TextRuntime.fs	ConvertDateOnly, ConvertTimeOnly, back-conversion methods
StructuralTypes.fs	DateOnly, TimeOnly cases in InferedTypeTag DU
StructuralInference.fs	DateOnly in primitiveTypes, typeTag, conversionTable, nameToType, matchValue; TimeOnly in nameToType only
ConversionsGenerator.fs	DateOnly/TimeOnly quotation branches
JsonConversionsGenerator.fs	DateOnly/TimeOnly quotation branches
JsonConversions.fs	AsDateOnly, AsTimeOnly
JsonRuntime.fs	ConvertDateOnly, ConvertTimeOnly, match cases
CsvInference.fs	dateonly?, timeonly?, dateonly option, timeonly option schema names
Test files	Updated DateTime → DateOnly for date-only values

Design decisions

1. No TimeOnly auto-inference — avoids breaking all existing TimeSpan-inferred code. Explicit annotation is unambiguous and sufficient for the use cases described in Support DateOnly and TimeOnly types #1461.
2. DateOnly before DateTime in matchValue — AsDateOnly is tried before AsDateTime so pure date strings resolve to DateOnly. AsDateTimeOffset is tried first (unchanged).
3. Subtype relationship preserved — DateOnly ⊆ DateTime, TimeOnly ⊆ TimeSpan. Mixed columns behave the same as before.

Test Status

• ✅ Build: dotnet build FSharp.Data.sln -c Release -maxcpucount:1 — succeeded
• ✅ FSharp.Data.Core.Tests: 2843 passed, 1 skipped — pre-existing network failure (HTTP 403 from google.com in sandbox)
• ✅ FSharp.Data.Tests: 248 passed, 0 failed

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[Copilot]

@dsyme I've opened a new pull request, #1610, to work on those changes. Once the pull request is ready, I'll request review from you.

[dsyme]

@copilot There is a problem in the design time tests:

Error: System.Exception : The design-time type 'System.DateOnly' utilized by a type provider was not found in the target reference assembly set '[tgt assembly FSharp.Data, Version=6.7.0.0, Culture=neutral, PublicKeyToken=49286adf818aa259;

See failing CI run

[Copilot]

@dsyme I've opened a new pull request, #1611, to work on those changes. Once the pull request is ready, I'll request review from you.