Address potential preview warnings (#378)

* Address potential preview warnings

We currently allow folks to use `prepareDependencies` in the body of a
preview, which works great. It does appear that previews have gotten
fancier, especially in Xcode 26, where a preview session stays live even
during changes to view code. These changes cause the preview body to
re-evaluate, triggering `prepareDependencies` all over again, and
spitting out a large purple warning into the preview console.

The behavior of `prepareDependencies` is to ignore future calls and
surface warnings in to catch misuse, but maybe we should only emit these
warnings in the application. Preview environments are a bit treacherous
as is, with app entry point code leaking into them, and preview trait
code executing across multiple previews in a file. And so this branch
proposes silencing this warning in previews entirely.

Alternatives:

- We could shorten the warning to a one-liner. Something like "Xcode
  Preview: 'prepareDependencies' already called this session. Ignoring."
- We could stop showing 'prepareDependencies' as a legit tool for
  previews and instead push people towards 'withDependencies', though
  this is not great for ergonomics, especially in non-single entry point
  systems.

Bonus change:

This PR also updates the warning about ignoring app entry point
dependencies in previews with instructions on how to suppress. But given
the above, maybe we actually want to suppress this warning entirely?

* Remove preview warning

It's not super helpful in practice.

Author: stephencelis

Sources/Dependencies/DependencyValues.swift

@@ -286,7 +286,6 @@ public struct DependencyValues: Sendable {
       if DependencyValues.isPreparing {
         #if canImport(SwiftUI)
           if context == .preview, Thread.isPreviewAppEntryPoint {
-            reportIssue("Ignoring dependencies prepared in preview app entry point")
             return
           }
         #endif
@@ -295,56 +294,58 @@ public struct DependencyValues: Sendable {
         let cacheKey = CachedValues.CacheKey(id: TypeIdentifier(key), context: context)
         guard !cachedValues.cached.keys.contains(cacheKey) else {
           if cachedValues.cached[cacheKey]?.preparationID != DependencyValues.preparationID {
-            reportIssue(
-              {
-                var dependencyDescription = ""
-                if let fileID = DependencyValues.currentDependency.fileID,
-                  let line = DependencyValues.currentDependency.line
+            if context != .preview {
+              reportIssue(
                 {
+                  var dependencyDescription = ""
+                  if let fileID = DependencyValues.currentDependency.fileID,
+                     let line = DependencyValues.currentDependency.line
+                  {
+                    dependencyDescription.append(
+                      """
+                        Location:
+                          \(fileID):\(line)
+
+                      """
+                    )
+                  }
                   dependencyDescription.append(
-                    """
-                      Location:
-                        \(fileID):\(line)
-
-                    """
-                  )
-                }
-                dependencyDescription.append(
-                  Key.self == Key.Value.self
+                    Key.self == Key.Value.self
                     ? """
-                      Dependency:
-                        \(typeName(Key.Value.self))
-                    """
+                        Dependency:
+                          \(typeName(Key.Value.self))
+                      """
                     : """
-                      Key:
-                        \(typeName(Key.self))
-                      Value:
-                        \(typeName(Key.Value.self))
-                    """
-                )
-                var argument: String {
-                  "\(function)" == "subscript(key:)"
+                        Key:
+                          \(typeName(Key.self))
+                        Value:
+                          \(typeName(Key.Value.self))
+                      """
+                  )
+                  var argument: String {
+                    "\(function)" == "subscript(key:)"
                     ? "\(typeName(Key.self)).self"
                     : "\\.\(function)"
-                }
-                return """
-                  @Dependency(\(argument)) has already been accessed or prepared.
+                  }
+                  return """
+                    @Dependency(\(argument)) has already been accessed or prepared.
 
-                  \(dependencyDescription)
+                    \(dependencyDescription)
 
-                  A global dependency can only be prepared a single time and cannot be accessed \
-                  beforehand. Prepare dependencies as early as possible in the lifecycle of your \
-                  application.
+                    A global dependency can only be prepared a single time and cannot be accessed \
+                    beforehand. Prepare dependencies as early as possible in the lifecycle of your \
+                    application.
 
-                  To temporarily override a dependency in your application, use 'withDependencies' \
-                  to do so in a well-defined scope.
-                  """
-              }(),
-              fileID: DependencyValues.currentDependency.fileID ?? fileID,
-              filePath: DependencyValues.currentDependency.filePath ?? filePath,
-              line: DependencyValues.currentDependency.line ?? line,
-              column: DependencyValues.currentDependency.column ?? column
-            )
+                    To temporarily override a dependency in your application, use \
+                    'withDependencies' to do so in a well-defined scope.
+                    """
+                }(),
+                fileID: DependencyValues.currentDependency.fileID ?? fileID,
+                filePath: DependencyValues.currentDependency.filePath ?? filePath,
+                line: DependencyValues.currentDependency.line ?? line,
+                column: DependencyValues.currentDependency.column ?? column
+              )
+            }
           } else {
             cachedValues.cached[cacheKey] = CachedValues.CachedValue(
               base: newValue,