refactor(vertexai): Split into separate libraries (#12794)

* Split into separate libraries

The current approach uses `part` files which cannot be imported
separately. One drawback of this approach is that any members which need
to be referenced by test code must be public to the library. Using
`src/` libraries, as opposed to `src/` part files, allows making APIs
that are "package private" by convention - they are public to the
library, but only exported through the `src/` import.

One potential downside (alternatively seen as a benefit) of using
separate libraries is that private members are truly private to their
library, and we cannot make a member of a class private to the package.
The convention for package imports only allows top level declarations to
be considered non-public implementation details.

- Remove imports from the top level library. Replace `part` lines with
  `export` of the same files.
- Add explicit `show` clauses to the exports. This is what allows an API
  to be public when imported from the `src/` library, but not exposed
  through the package public library.
- Change some private `_toGoogleAI<TypeName>` methods to extension
  methods. Use the name `toGoogleAI` since the specific type names are
  clear from context.
- Change some private `_fromGoogleAI<TypeName>` factory constructors to
  extension methods on the google AI SDk types. Name `toVertex()`.
- Remove some unused private methods that were filled in for
  consistency. More extension members can be added as needed.
- Add an extension to expose the private field `_gooleAiModel` from
  `GenerativeModel`.
- Add a top level method to forward to the private `GenerativeModel`
  constructor.
- Add TODO comments to stop exporting some methods that don't need to be
  public. These could have been private in the `part` pattern. For now
  they are exported for backwards compatibility, and we can remove them
  when we are ready for a major version bump.

* Fix typo

* Add (redundant) docs

* dart format

* Add missing auth argument forwarding

* Bad merge?

* More bad merge

* Unnecessary change

* Update packages/firebase_vertexai/firebase_vertexai/lib/src/vertex_api.dart

Co-authored-by: Russell Wheatley <russellwheatley85@gmail.com>

---------

Co-authored-by: Cynthia J <cynthiajoan@users.noreply.github.com>
Co-authored-by: Russell Wheatley <russellwheatley85@gmail.com>

packages/firebase_vertexai/firebase_vertexai/lib/firebase_vertexai.dart

@@ -12,26 +12,58 @@
 // See the License for the specific language governing permissions and
 // limitations under the License.
 
-library firebase_vertexai;
-
-import 'dart:async';
-import 'dart:convert';
-import 'dart:typed_data';
-
-import 'package:firebase_app_check/firebase_app_check.dart';
-import 'package:firebase_auth/firebase_auth.dart';
-import 'package:firebase_core/firebase_core.dart';
-import 'package:firebase_core_platform_interface/firebase_core_platform_interface.dart'
-    show FirebasePluginPlatform;
-import 'package:google_generative_ai/google_generative_ai.dart' as google_ai;
-// ignore: implementation_imports, tightly coupled packages
-import 'package:google_generative_ai/src/vertex_hooks.dart';
-
-import 'src/vertex_version.dart';
-
-part 'src/firebase_vertexai.dart';
-part 'src/vertex_api.dart';
-part 'src/vertex_chat.dart';
-part 'src/vertex_content.dart';
-part 'src/vertex_function_calling.dart';
-part 'src/vertex_model.dart';
+export 'src/firebase_vertexai.dart'
+    show
+        // TODO(next breaking): Remove defaultTimeout
+        defaultTimeout,
+        FirebaseVertexAI,
+        RequestOptions;
+export 'src/vertex_api.dart'
+    show
+        BatchEmbedContentsResponse,
+        BlockReason,
+        Candidate,
+        CitationMetadata,
+        CitationSource,
+        ContentEmbedding,
+        CountTokensResponse,
+        // TODO(next breaking): Remove CountTokensResponseFields
+        CountTokensResponseFields,
+        EmbedContentRequest,
+        EmbedContentResponse,
+        FinishReason,
+        GenerateContentResponse,
+        GenerationConfig,
+        HarmBlockThreshold,
+        HarmCategory,
+        HarmProbability,
+        PromptFeedback,
+        SafetyRating,
+        SafetySetting,
+        TaskType,
+        // TODO(next breaking): Remove parse* methods
+        parseCountTokensResponse,
+        parseEmbedContentResponse,
+        parseGenerateContentResponse;
+export 'src/vertex_chat.dart' show ChatSession, StartChatExtension;
+export 'src/vertex_content.dart'
+    show
+        Content,
+        DataPart,
+        FileData,
+        FunctionCall,
+        FunctionResponse,
+        Part,
+        TextPart,
+        // TODO(next breaking): Remove parseContent
+        parseContent;
+export 'src/vertex_function_calling.dart'
+    show
+        FunctionCallingConfig,
+        FunctionCallingMode,
+        FunctionDeclaration,
+        Schema,
+        SchemaType,
+        Tool,
+        ToolConfig;
+export 'src/vertex_model.dart' show GenerativeModel;

packages/firebase_vertexai/firebase_vertexai/lib/src/firebase_vertexai.dart

@@ -12,7 +12,16 @@
 // See the License for the specific language governing permissions and
 // limitations under the License.
 
-part of firebase_vertexai;
+import 'package:firebase_app_check/firebase_app_check.dart';
+import 'package:firebase_auth/firebase_auth.dart';
+import 'package:firebase_core/firebase_core.dart';
+import 'package:firebase_core_platform_interface/firebase_core_platform_interface.dart'
+    show FirebasePluginPlatform;
+
+import 'vertex_api.dart';
+import 'vertex_content.dart';
+import 'vertex_function_calling.dart';
+import 'vertex_model.dart';
 
 const _defaultLocation = 'us-central1';
 
@@ -105,7 +114,7 @@ class FirebaseVertexAI extends FirebasePluginPlatform {
       Content? systemInstruction,
       List<Tool>? tools,
       ToolConfig? toolConfig}) {
-    return GenerativeModel._(
+    return createGenerativeModel(
         model: model,
         app: app,
         appCheck: appCheck,