fix: remove once in ftl.Map to allow changing underlying values (#1435)

fixes #1390

`ftl.Map` was just using `once` as an optimisation.
But this unit test case was not being handled properly:
```
ctx := ftltest.Context(
    ftltest.WithConfig(example, "hello"),
)

//Call a verb that uses ftl.Map with example config
resp, err := Verb(ctx, req)

ctx = ftltest.Context(
    ftltest.WithConfig(example, "world"),
)

//When this execution of the verb is done, it should execute again because the underlying value is different (hello -> world)
resp, err = Verb(ctx, req)
```
To solve this, now the mapping func is called each time.

For use cases where modules want to map a `*sql.DB` to a `DAL`, it will
create a new DAL instance each time

go-runtime/ftl/map.go

@@ -2,28 +2,22 @@ package ftl
 
 import (
 	"context"
-	"sync"
 )
 
 type MapHandle[T, U any] struct {
 	fn     func(context.Context, T) (U, error)
 	getter Handle[T]
-	out    U
-	once   *sync.Once
 }
 
-func (sh *MapHandle[T, U]) Get(ctx context.Context) U {
-	sh.once.Do(func() {
-		t, err := sh.fn(ctx, sh.getter.Get(ctx))
-		if err != nil {
-			panic(err)
-		}
-		sh.out = t
-	})
-	return sh.out
+func (mh *MapHandle[T, U]) Get(ctx context.Context) U {
+	t, err := mh.fn(ctx, mh.getter.Get(ctx))
+	if err != nil {
+		panic(err)
+	}
+	return t
 }
 
 // Map an FTL resource type to a new type.
 func Map[T, U any](getter Handle[T], fn func(context.Context, T) (U, error)) MapHandle[T, U] {
-	return MapHandle[T, U]{fn: fn, getter: getter, once: &sync.Once{}}
+	return MapHandle[T, U]{fn: fn, getter: getter}
 }

go-runtime/ftl/map_test.go

@@ -12,21 +12,6 @@ type intHandle int
 
 func (s intHandle) Get(ctx context.Context) int { return int(s) }
 
-func TestMapBaseCase(t *testing.T) {
-	incrementer := 0
-
-	h := intHandle(123)
-	ctx := context.Background()
-	once := Map(h, func(ctx context.Context, n int) (string, error) {
-		incrementer++
-		return fmt.Sprintf("handle: %d", n), nil
-	})
-
-	assert.Equal(t, once.Get(ctx), "handle: 123")
-	assert.Equal(t, once.Get(ctx), "handle: 123")
-	assert.Equal(t, incrementer, 1)
-}
-
 func TestMapPanic(t *testing.T) {
 	ctx := context.Background()
 	n := intHandle(1)