Fix intermittent error ("can't add a new key into hash during iteration") in the theme push command

[karreiro]

WHY are these changes introduced?

The error "can't add a new key into hash during iteration" appears intermittently when users run shopify theme push.

WHAT is this pull request doing?

The error mentioned in the #1705 issue happens at the update_checksums method (in the ShopifyCLI::Theme::Syncer class):

def update_checksums(api_response)
  api_response.values.flatten.each do |asset|
    if asset["key"]
      @checksums[asset["key"]] = asset["checksum"] # <-------------------------- It's raised here
    end
  end

  # [...]
  @checksums.reject! { |key, _| @checksums.key?("#{key}.liquid") } # <---------- Notice
end

Here's a simpler reproducer of the same error (with multiple threads modifying with the same Hash):

hash = {}

modify_hash = -> do
  hash[rand(1..99).to_s] = rand(1..99).to_s
  hash.reject! { |k, _v| k == rand(1..99).to_s }
end

t1 = Thread.new do
  loop do
    sleep(0.00005 * 1)
    modify_hash.call
  end
end

t2 = Thread.new do
  loop do
    sleep(0.00005 * 2)
    modify_hash.call
  end
end

t3 = Thread.new do
  loop do
    sleep(0.00005 * 3)
    modify_hash.call
  end
end

t1.join
t2.join
t3.join

This PR adds a semaphore to coordinate changes in the @checksums object (shared across multiple threads).

How to test your changes?

• Run shopify-dev theme push -u -t <name> many times with different names in the -t flag
• Notice the intermittent error no longer happens

Post-release steps

None.

Update checklist

• I've added a CHANGELOG entry for this PR (if the change is public-facing)
• I've considered possible cross-platform impacts (Mac, Linux, Windows).
• I've left the version number as is (we'll handle incrementing this when releasing).
• I've included any post-release steps in the section above.

[macournoyer]

Amazing! Nice catch 👏