docs: salvage F# agent and language guidance

rules/fsharp/testing.md

@@ -0,0 +1,62 @@
+---
+paths:
+  - "**/*.fs"
+  - "**/*.fsx"
+  - "**/*.fsproj"
+---
+# F# Testing
+
+> This file extends [common/testing.md](../common/testing.md) with F#-specific content.
+
+## Test Framework
+
+- Prefer **xUnit** with **FsUnit.xUnit** for F#-friendly assertions
+- Use **Unquote** for quotation-based assertions with clear failure messages
+- Use **FsCheck.xUnit** for property-based testing
+- Use **NSubstitute** or function stubs for mocking dependencies
+- Use **Testcontainers** when integration tests need real infrastructure
+
+## Test Organization
+
+- Mirror `src/` structure under `tests/`
+- Separate unit, integration, and end-to-end coverage clearly
+- Name tests by behavior, not implementation details
+
+```fsharp
+open Xunit
+open Swensen.Unquote
+
+[<Fact>]
+let ``PlaceOrder returns success when request is valid`` () =
+    let request = { CustomerId = "cust-123"; Items = [ validItem ] }
+    let result = OrderService.placeOrder request
+    test <@ Result.isOk result @>
+
+[<Fact>]
+let ``PlaceOrder returns error when items are empty`` () =
+    let request = { CustomerId = "cust-123"; Items = [] }
+    let result = OrderService.placeOrder request
+    test <@ Result.isError result @>
+```
+
+## Property-Based Testing with FsCheck
+
+```fsharp
+open FsCheck.Xunit
+
+[<Property>]
+let ``order total is never negative`` (items: OrderItem list) =
+    let total = Order.calculateTotal items
+    total >= 0m
+```
+
+## ASP.NET Core Integration Tests
+
+- Use `WebApplicationFactory<TEntryPoint>` for API integration coverage
+- Test auth, validation, and serialization through HTTP, not by bypassing middleware
+
+## Coverage
+
+- Target 80%+ line coverage
+- Focus coverage on domain logic, validation, auth, and failure paths
+- Run `dotnet test` in CI with coverage collection enabled where available