Use JSON.generate() instead to improve performance

It seems it is more efficient using JSON.generate() instead of the #to_json method.

### benchmark
```ruby
require 'bundler/inline'
gemfile do
  source 'https://rubygems.org'
  gem 'benchmark-ips'
  gem 'json'
end

json =<<~JSON
{
  "method": "$/steep/typecheck/progress",
  "params": {
    "guid": "8d4c15ce-33ad-40d8-bef4-8713f29cab5b",
    "path": "/home/watson/prj/gruff/lib/gruff/store/basic_data.rb",
    "target": "lib",
    "diagnostics": []
  },
  "jsonrpc": "2.0"
}
JSON

data = JSON.parse(json)

puts "** JSON version #{JSON::VERSION} **"

Benchmark.ips do |x|
  x.report('to_json') { data.to_json }
  x.report('JSON.generate') { JSON.generate(data) }

  x.compare!
end
```

### result
```sh
$ ruby json.rb
** JSON version 2.11.3 **
ruby 3.4.3 (2025-04-14 revision d0b7e5b6a0) +PRISM [x86_64-linux]
Warming up --------------------------------------
             to_json   293.738k i/100ms
       JSON.generate   335.385k i/100ms
Calculating -------------------------------------
             to_json      2.887M (± 1.4%) i/s  (346.41 ns/i) -     14.687M in   5.088761s
       JSON.generate      3.366M (± 1.0%) i/s  (297.10 ns/i) -     17.105M in   5.082281s

Comparison:
       JSON.generate:  3365882.0 i/s
             to_json:  2886757.9 i/s - 1.17x  slower
```

### environmet
- OS: Manjaro Linux x86_64
- Kernel: 6.14.4-1-MANJARO
- Compiler: gcc 14.2.1 20250207
- Ruby: ruby 3.4.3

Author: Watson1978

lib/language_server/protocol/transport/io/writer.rb

@@ -11,9 +11,9 @@ def initialize(io)
           end
 
           def write(response)
-            response_str = response.merge(
+            response_str = JSON.generate(response.merge(
               jsonrpc: "2.0"
-            ).to_json
+            ))
 
             headers = {
               "Content-Length" => response_str.bytesize