Support analyzer 13 in gql codegen

[knaeckeKami]

Summary

• Widen gql_code_builder and gql_build analyzer constraints to support analyzer 13 while keeping the existing >=9.0.0 analyzer minimum.
• Add a minimum-dependency verification script that runs full dart pub downgrade in temp package copies, analyzes both codegen packages, and runs tests when present.
• Add a dev dependency floor for frontend_server_client so downgraded gql_code_builder tests run on the current Dart SDK.
• Wire the minimum dependency check into Melos and PR validation CI.

Testing

• ./tool/verify_min_dependencies.sh
• dart run melos run verify-min-dependencies
• Clean temp latest-resolution check with analyzer 13.3.0 for gql_code_builder and gql_build
• dart test in codegen/gql_code_builder