fix: let LogBatcherParser fall back to OPENAI_API_KEY

[viktorbeck98]

Task

Closes #356

Description

LogBatcherParser passed its api_key setting straight to the OpenAI client. The default is an empty string, and the client only reads OPENAI_API_KEY when api_key is None. So the environment variable was never used, and LogBatcherParser() failed on creation unless the key was written into the config.

An empty api_key is now passed as None, the same way base_url already is. A key set in the config still takes precedence over the environment variable.

Not changed here: the docs example still says to put the key in the YAML.

How Has This Been Tested?

• 2 new tests: the environment fallback, which fails before the fix with the error from the issue, and config-over-environment precedence. Full suite passes.
• The snippet from the issue now creates the parser.

prek ran locally but skipped both changed files. The top-level exclude: '.*.log' in .pre-commit-config.yaml also matches paths that only contain "log", such as parsers/logbatcher/, so no linter checked this change.

Checklist

• This Pull-Request goes to the development branch.
• I have successfully run prek locally.
• I have added tests to cover my changes.
• I have linked the issue-id to the task-description.
• I have performed a self-review of my own code.

---

^{Need help on this PR? Tag @codesmith-bot with what you need. Autofix is disabled.}