fix(logging): support standard OPENAI_LOG levels (#3734)

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## Changes being requested

Fixes #3096.

`OPENAI_LOG` currently recognizes only `debug` and `info`. This change
supports the standard logging levels `debug`, `info`, `warning`,
`error`, and `critical` through a single level mapping, while preserving
the existing behavior for unset or unknown values.

This lets applications suppress routine SDK request logs without having
to reconfigure the `openai` logger in application code.

## Additional context & links

Adds regression coverage asserting that each supported `OPENAI_LOG`
value configures the `openai` logger at the corresponding
standard-library logging level.

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Co-authored-by: Marcus Wood <marcuswood@openai.com>