fix cognitive service tests failure

[amber-Chen-86]

Description

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• The pull request does not introduce [breaking changes]
• CHANGELOG is updated for new features, bug fixes or other significant changes.
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Testing Guidelines

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[Copilot]

Pull request overview

This PR adds test fixtures for environment variables to fix failing tests in two Azure Cognitive Language Service packages. The changes standardize the test configuration to properly load endpoint and API key values from environment variables during test execution.

• Adds fixtures for retrieving test endpoint and API key values from environment variables
• Updates comments to be more concise and accurate

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated no comments.

File	Description
sdk/cognitivelanguage/azure-ai-textanalytics-authoring/tests/conftest.py	Adds authoring_endpoint and authoring_key fixtures to load test credentials from environment variables
sdk/cognitivelanguage/azure-ai-language-conversations/tests/conftest.py	Adds conversations_endpoint and conversations_key fixtures to load test credentials from environment variables