fix: consistent env var parsing for all boolean args in vLLM

[tjohnson31415]

Support consistent parsing of boolean arguments defined in vLLM.

Description

In vLLM, some args use action='store_true':
https://github.com/vllm-project/vllm/blob/df845b2b46c3e30f5bd3e3be286285ed148323fc/vllm/engine/arg_utils.py#L202-L204
and --enable-chunked-prefill has a custom arg parsing action called StoreBoolean:
https://github.com/vllm-project/vllm/blob/df845b2b46c3e30f5bd3e3be286285ed148323fc/vllm/engine/arg_utils.py#L561-L568

We want EnvVarArgumentParser to treat all of these boolean args consistently. The fix in this PR is to inspect the type of the action in addition to action.type to detect when we need to parse a boolean from the env var value.

In my testing, I also learned that argparse will parse default string values into the correct type for the argument, so there's no need to have special handling for int args (which would also imply special handling of float, _bool_from_string, nullable_str, and other typed args).

How Has This Been Tested?

Tested by hand for the actual vLLM args and also added unit tests of EnvVarArgumentParser for the 'store_true', 'store_false', and StoreBoolean action types for boolean args.

Merge criteria:

• The commits are squashed in a cohesive manner and have meaningful messages.
• Testing instructions have been added in the PR body (for PRs involving changes that are not immediately obvious).
• The developer has manually tested the changes and verified that the changes work

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 55.87%. Comparing base (50b020b) to head (54469cf).

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #98      +/-   ##
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+ Coverage   54.77%   55.87%   +1.09%     
==========================================
  Files          24       24              
  Lines        1446     1482      +36     
  Branches      259      266       +7     
==========================================
+ Hits          792      828      +36     
  Misses        579      579              
  Partials       75       75              

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[njhill]

Thanks @tjohnson31415!