Automatic retry logic

[tombch]

• Addresses Automatic retry logic in Python API #121
• Adds automatic request retries with exponential backoff for status codes 502, 503 and 504.

[Copilot]

Pull request overview

Adds automatic retry behavior to the Python Onyx client so transient gateway/service failures (502/503/504) are retried with exponential backoff, addressing #121.

Changes:

• Configure a requests.Session with an HTTPAdapter + Retry strategy and use it both inside and outside the client context manager.
• Add unit tests asserting retry adapter configuration and verifying retry behavior.
• Add responses to the dev/test optional dependency groups.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated 5 comments.

File	Description
onyx/core.py	Introduces retry constants and configures a retry-enabled requests.Session; updates context manager behavior to use the configured session.
tests/test_api.py	Adds tests to validate retry configuration and runtime retry behavior.
pyproject.toml	Adds responses as a test/dev dependency to support the new retry tests.

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[tombch]

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