Add comprehensive tests for CSV extension methods AsTimeSpan and AsDateTimeOffset

- Add 10 new test cases covering AsTimeSpan() and AsDateTimeOffset() extension methods in CSV context
- Test valid inputs with different time formats (simple, complex, with milliseconds)
- Test DateTimeOffset parsing with positive, negative, and zero UTC offsets
- Test error handling for invalid input strings
- Test culture-specific parsing scenarios
- Test integration with CSV dynamic operator (?)
- Addresses test coverage gap identified for these extension methods

This improves test coverage for CSV extension methods that were previously untested
in the CSV context, even though the underlying functionality was tested elsewhere.

Total test count increased from 2393 to 2403 tests, all passing.

🤖 Generated with [Claude Code](https://claude.ai/code)

Co-Authored-By: Claude <noreply@anthropic.com>

Author: claude

tests/FSharp.Data.Core.Tests/CsvFile.fs

@@ -262,4 +262,94 @@ let ``Complex CSV with quotes and newlines handled correctly`` () =
     rows.[0].["Name"] |> should equal "Alice"
     rows.[0].["Description"] |> should equal "Software Engineer, Senior"
     rows.[1].["Description"] |> should equal "Product Manager\"s Assistant"
-    rows.[2].["Description"] |> should equal "Data Scientist\nwith ML focus"
+    rows.[2].["Description"] |> should equal "Data Scientist\nwith ML focus"
+
+// Sample CSV data with time/date fields for testing extension methods
+let csvWithDateTimeData = """StartDate,EndDate,Duration,Offset
+2023-01-15,2023-01-20,5.00:00:00,2023-01-15T10:30:00+02:00
+2023-02-01,2023-02-03,2.12:30:45,2023-02-01T14:15:30-05:00
+2023-03-10,2023-03-15,4.08:15:20,2023-03-10T09:45:00+00:00"""
+
+// StringExtensions tests for CSV context - Missing coverage area
+[<Test>]
+let ``StringExtensions.AsTimeSpan works with valid input`` () =
+    let csv = CsvFile.Parse(csvWithDateTimeData)
+    let firstRow = csv.Rows |> Seq.head
+    
+    firstRow.["Duration"].AsTimeSpan() |> should equal (System.TimeSpan(5, 0, 0, 0))
+
+[<Test>]
+let ``StringExtensions.AsTimeSpan works with complex time format`` () =
+    let csv = CsvFile.Parse(csvWithDateTimeData)
+    let rows = csv.Rows |> Array.ofSeq
+    
+    rows.[1].["Duration"].AsTimeSpan() |> should equal (System.TimeSpan(2, 12, 30, 45))
+    rows.[2].["Duration"].AsTimeSpan() |> should equal (System.TimeSpan(4, 8, 15, 20))
+
+[<Test>]
+let ``StringExtensions.AsTimeSpan throws with invalid input`` () =
+    let csv = CsvFile.Parse("InvalidTime\ninvalid_time")
+    let row = csv.Rows |> Seq.head
+    
+    Assert.Throws<System.Exception>(fun () -> row.["InvalidTime"].AsTimeSpan() |> ignore) |> ignore
+
+[<Test>]
+let ``StringExtensions.AsDateTimeOffset works with valid input`` () =
+    let csv = CsvFile.Parse(csvWithDateTimeData)
+    let firstRow = csv.Rows |> Seq.head
+    
+    let result = firstRow.["Offset"].AsDateTimeOffset()
+    result.DateTime |> should equal (System.DateTime(2023, 1, 15, 10, 30, 0))
+    result.Offset |> should equal (System.TimeSpan(2, 0, 0))
+
+[<Test>]
+let ``StringExtensions.AsDateTimeOffset works with negative offset`` () =
+    let csv = CsvFile.Parse(csvWithDateTimeData)
+    let rows = csv.Rows |> Array.ofSeq
+    
+    let result = rows.[1].["Offset"].AsDateTimeOffset()
+    result.DateTime |> should equal (System.DateTime(2023, 2, 1, 14, 15, 30))
+    result.Offset |> should equal (System.TimeSpan(-5, 0, 0))
+
+[<Test>]
+let ``StringExtensions.AsDateTimeOffset works with zero offset`` () =
+    let csv = CsvFile.Parse(csvWithDateTimeData)
+    let rows = csv.Rows |> Array.ofSeq
+    
+    let result = rows.[2].["Offset"].AsDateTimeOffset()
+    result.DateTime |> should equal (System.DateTime(2023, 3, 10, 9, 45, 0))
+    result.Offset |> should equal (System.TimeSpan.Zero)
+
+[<Test>]
+let ``StringExtensions.AsDateTimeOffset throws with invalid input`` () =
+    let csv = CsvFile.Parse("InvalidOffset\ninvalid_offset")
+    let row = csv.Rows |> Seq.head
+    
+    Assert.Throws<System.Exception>(fun () -> row.["InvalidOffset"].AsDateTimeOffset() |> ignore) |> ignore
+
+[<Test>]
+let ``StringExtensions.AsTimeSpan with custom culture`` () =
+    let csv = CsvFile.Parse("Duration\n01:30:45")
+    let row = csv.Rows |> Seq.head
+    
+    let result = row.["Duration"].AsTimeSpan(System.Globalization.CultureInfo.InvariantCulture)
+    result |> should equal (System.TimeSpan(1, 30, 45))
+
+[<Test>]
+let ``StringExtensions.AsDateTimeOffset with custom culture`` () =
+    let csv = CsvFile.Parse("Timestamp\n2023-06-15T16:20:30+03:00")
+    let row = csv.Rows |> Seq.head
+    
+    let result = row.["Timestamp"].AsDateTimeOffset(System.Globalization.CultureInfo.InvariantCulture)
+    result.DateTime |> should equal (System.DateTime(2023, 6, 15, 16, 20, 30))
+    result.Offset |> should equal (System.TimeSpan(3, 0, 0))
+
+[<Test>]
+let ``StringExtensions methods work with dynamic operator`` () =
+    let csv = CsvFile.Parse(csvWithDateTimeData)
+    let row = csv.Rows |> Seq.head
+    
+    // Test that the ? operator integrates properly with extension methods
+    row?Duration.AsTimeSpan() |> should equal (System.TimeSpan(5, 0, 0, 0))
+    let offsetResult = row?Offset.AsDateTimeOffset()
+    offsetResult.Offset |> should equal (System.TimeSpan(2, 0, 0))