build: add Coverage FAKE target for code coverage collection

Uses the Coverlet packages already present in FSharp.Data.Core.Tests
(coverlet.collector 6.0.4 + coverlet.msbuild 6.0.4) to collect XPlat
Code Coverage during the test run, writing results to ./coverage-results/.

Adds user-friendly guidance for generating an HTML report with
dotnet-reportgenerator-globaltool (no new required dependency).

Usage:
  dotnet run --project build/build.fsproj -t Coverage

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: github-actions[bot]

build/build.fs

@@ -123,6 +123,32 @@ let buildscript () =
 
         "FSharp.Data.sln" |> DotNet.test setParams)
 
+    // --------------------------------------------------------------------------------------
+    // Run tests with code coverage (uses Coverlet collector already present in test projects)
+
+    Target.create "Coverage" (fun _ ->
+        let setParams (o: DotNet.TestOptions) =
+            { o with
+                Configuration = DotNet.BuildConfiguration.Release
+                Collect = Some "XPlat Code Coverage"
+                ResultsDirectory = Some "coverage-results"
+                MSBuildParams =
+                    { o.MSBuildParams with
+                        DisableInternalBinLog = true }
+                Logger = if isCI then Some "GitHubActions" else None }
+
+        "FSharp.Data.sln" |> DotNet.test setParams
+
+        Trace.log ""
+        Trace.log "Coverage results written to ./coverage-results/"
+        Trace.log "To generate an HTML report, install dotnet-reportgenerator-globaltool:"
+        Trace.log "  dotnet tool install -g dotnet-reportgenerator-globaltool"
+
+        Trace.log
+            "  reportgenerator -reports:coverage-results/**/coverage.cobertura.xml -targetdir:coverage-report -reporttypes:Html"
+
+        Trace.log "")
+
     // --------------------------------------------------------------------------------------
     // Build packages
 
@@ -179,6 +205,7 @@ let buildscript () =
         printfn "  Targets for building:"
         printfn "  * Build"
         printfn "  * RunTests"
+        printfn "  * Coverage (run tests with Coverlet code coverage; results in ./coverage-results/)"
         printfn "  * GenerateDocs"
         printfn "  * Pack (creates package only, doesn't publish)"
         printfn "  * All (calls previous 5)"
@@ -247,6 +274,7 @@ let buildscript () =
     "Build" ==> "Pack" ==> "All"
     "Build" ==> "All"
     "Build" ==> "RunTests" ==> "All"
+    "Build" ==> "Coverage"
     "Build" ==> "RunBenchmarks"
 
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