feat: migrate to analyzer 9.0.0 API

- Replace Element2 API with Element API
- Update imports from element2.dart to element.dart
- Replace .firstFragment.name2 with .name
- Replace .firstFragment.metadata2.annotations with .metadata.annotations
- Replace .fields2 with .fields
- Replace .methods2 with .methods
- Remove duplicate imports in annotation_validator.dart and generated_class.dart
- Update test mocks to use Element instead of Element2

All generators now work correctly with analyzer 9.0.0:
- melos analyze: SUCCESS (0 issues)
- melos codegen: SUCCESS (13 outputs generated)
- melos bootstrap: SUCCESS (8 packages)

examples/client_app/pubspec.lock

@@ -5,18 +5,18 @@ packages:
     dependency: transitive
     description:
       name: _fe_analyzer_shared
-      sha256: c209688d9f5a5f26b2fb47a188131a6fb9e876ae9e47af3737c0b4f58a93470d
+      sha256: "5b7468c326d2f8a4f630056404ca0d291ade42918f4a3c6233618e724f39da8e"
       url: "https://pub.dev"
     source: hosted
-    version: "91.0.0"
+    version: "92.0.0"
   analyzer:
     dependency: transitive
     description:
       name: analyzer
-      sha256: f51c8499b35f9b26820cfe914828a6a98a94efd5cc78b37bb7d03debae3a1d08
+      sha256: "70e4b1ef8003c64793a9e268a551a82869a8a96f39deb73dea28084b0e8bf75e"
       url: "https://pub.dev"
     source: hosted
-    version: "8.4.1"
+    version: "9.0.0"
   args:
     dependency: transitive
     description:
@@ -45,10 +45,10 @@ packages:
     dependency: transitive
     description:
       name: build
-      sha256: ce76b1d48875e3233fde17717c23d1f60a91cc631597e49a400c89b475395b1d
+      sha256: "275bf6bb2a00a9852c28d4e0b410da1d833a734d57d39d44f94bfc895a484ec3"
       url: "https://pub.dev"
     source: hosted
-    version: "3.1.0"
+    version: "4.0.4"
   build_config:
     dependency: transitive
     description:
@@ -65,30 +65,14 @@ packages:
       url: "https://pub.dev"
     source: hosted
     version: "4.1.1"
-  build_resolvers:
-    dependency: transitive
-    description:
-      name: build_resolvers
-      sha256: d1d57f7807debd7349b4726a19fd32ec8bc177c71ad0febf91a20f84cd2d4b46
-      url: "https://pub.dev"
-    source: hosted
-    version: "3.0.3"
   build_runner:
     dependency: "direct dev"
     description:
       name: build_runner
-      sha256: b24597fceb695969d47025c958f3837f9f0122e237c6a22cb082a5ac66c3ca30
+      sha256: b4d854962a32fd9f8efc0b76f98214790b833af8b2e9b2df6bfc927c0415a072
       url: "https://pub.dev"
     source: hosted
-    version: "2.7.1"
-  build_runner_core:
-    dependency: transitive
-    description:
-      name: build_runner_core
-      sha256: "066dda7f73d8eb48ba630a55acb50c4a84a2e6b453b1cb4567f581729e794f7b"
-      url: "https://pub.dev"
-    source: hosted
-    version: "9.3.1"
+    version: "2.10.5"
   built_collection:
     dependency: transitive
     description:
@@ -134,7 +118,7 @@ packages:
       path: "../../cherrypick_annotations"
       relative: true
     source: path
-    version: "3.0.2"
+    version: "4.0.0-dev.0"
   cherrypick_flutter:
     dependency: "direct main"
     description:
@@ -148,7 +132,7 @@ packages:
       path: "../../cherrypick_generator"
       relative: true
     source: path
-    version: "3.0.2"
+    version: "4.0.0-dev.0"
   clock:
     dependency: transitive
     description:
@@ -247,14 +231,6 @@ packages:
     description: flutter
     source: sdk
     version: "0.0.0"
-  frontend_server_client:
-    dependency: transitive
-    description:
-      name: frontend_server_client
-      sha256: f64a0333a82f30b0cca061bc3d143813a486dc086b574bfb233b7c1372427694
-      url: "https://pub.dev"
-    source: hosted
-    version: "4.0.0"
   glob:
     dependency: transitive
     description:
@@ -500,14 +476,6 @@ packages:
       url: "https://pub.dev"
     source: hosted
     version: "0.7.7"
-  timing:
-    dependency: transitive
-    description:
-      name: timing
-      sha256: "62ee18aca144e4a9f29d212f5a4c6a053be252b895ab14b5821996cff4ed90fe"
-      url: "https://pub.dev"
-    source: hosted
-    version: "1.0.2"
   typed_data:
     dependency: transitive
     description: