Make background tasks lazy

[schneems]

Previously when a background task was defined it would perform work on initialization, this fails if the first task is not actually executed yet:

:::>- pre.erb background.start('heroku run:inside <%= dyno_ps_name %> "bash"', wait: "$", timeout: 120, name: "heroku_run")
:::-- background.stdin_write("ls -lah", name: "heroku_run", wait: "$", timeout: 30)
:::-- background.stdin_write("exit", name: "heroku_run", wait: "exit")
:::-> background.stop(name: "heroku_run")

Gives:

/Users/rschneeman/.gem/ruby/3.3.1/gems/rundoc-4.1.1/lib/rundoc/code_command/background/process_spawn.rb:41:in `find': Could not find task with name "heroku_run", known task names: [] (RuntimeError)
	from /Users/rschneeman/.gem/ruby/3.3.1/gems/rundoc-4.1.1/lib/rundoc/code_command/background/stdin_write.rb:9:in `initialize'
	from /Users/rschneeman/.gem/ruby/3.3.1/gems/rundoc-4.1.1/lib/rundoc.rb:13:in `new'
	from /Users/rschneeman/.gem/ruby/3.3.1/gems/rundoc-4.1.1/lib/rundoc.rb:13:in `code_command_from_keyword'
	from /Users/rschneeman/.gem/ruby/3.3.1/gems/rundoc-4.1.1/lib/rundoc/peg_parser.rb:210:in `block in <class:PegTransformer>'
	from /Users/rschneeman/.gem/ruby/3.3.1/gems/parslet-2.0.0/lib/parslet/transform.rb:217:in `instance_eval'
	from /Users/rschneeman/.gem/ruby/3.3.1/gems/parslet-2.0.0/lib/parslet/transform.rb:217:in `call_on_match'
	from /Users/rschneeman/.gem/ruby/3.3.1/gems/parslet-2.0.0/lib/parslet/transform.rb:235:in `block in transform_elt'
	from /Users/rschneeman/.gem/ruby/3.3.1/gems/parslet-2.0.0/lib/parslet/transform.rb:232:in `each'

Because the background.stdin_write call to new() happens at parse time, but the background.start call to new() happens at runtime, (Because pre.erb is the command, and it hasn't yet created the start command and pushed it onto the stack).

This change makes the lookup for background tasks lazy, so as long as the task is started by execution (runtime) of the command it will succeed.

The added test fails on main (with the error below) and passes on this branch.

  1) Error:
BackgroundTest#test_stdin_with_cat_echo:
RuntimeError: Could not find task with name "cat", known task names: ["script", "tail"]
    lib/rundoc/code_command/background/process_spawn.rb:41:in `find'
    lib/rundoc/code_command/background/stdin_write.rb:9:in `initialize'
    test/rundoc/code_commands/background_test.rb:9:in `new'
    test/rundoc/code_commands/background_test.rb:9:in `block (2 levels) in test_stdin_with_cat_echo'
    test/rundoc/code_commands/background_test.rb:6:in `chdir'
    test/rundoc/code_commands/background_test.rb:6:in `block in test_stdin_with_cat_echo'
    /Users/rschneeman/.rubies/ruby-3.3.1/lib/ruby/3.3.0/tmpdir.rb:99:in `mktmpdir'
    test/rundoc/code_commands/background_test.rb:5:in `test_stdin_with_cat_echo'

87 runs, 249 assertions, 0 failures, 1 errors, 0 skips