chore: Address warning: literal string will be frozen in the future

[yahonda]

This pull request addresses warning: literal string will be frozen in the future introduced since Ruby 3.4

Steps to reproduce

This will emit the warning: literal string will be frozen in the future warnings and shows the info: the string was created here to see where the string literal is created.

$ ruby -v
ruby 3.4.2 (2025-02-15 revision d2930f8e7a) +PRISM [x86_64-linux]
$ cd google-apis-core
$ RUBYOPT="--debug-frozen-string-literal" bundle exec rspec -w

Warnings addressed by this commit

/path/to/google-apis-core/lib/google/apis/core/composite_io.rb:45: info: the string was created here
/path/to/google-apis-core/lib/google/apis/core/download.rb:49: info: the string was created here
/path/to/google-apis-core/lib/google/apis/core/http_command.rb:286: info: the string was created here
/path/to/google-apis-core/lib/google/apis/core/http_command.rb:388: info: the string was created here
/path/to/google-apis-core/lib/google/apis/core/http_command.rb:396: info: the string was created here
/path/to/google-apis-core/lib/google/apis/core/multipart.rb:34: info: the string was created here
/path/to/google-apis-core/lib/google/apis/core/multipart.rb:62: info: the string was created here
/path/to/google-apis-core/lib/google/apis/errors.rb:46: info: the string was created here