Document hidden environment variable loading

[mccoyp]

Description

#44556 added support for shielding secret environment variable values. This documents how to use this API and warns against improper use (like storing the secret in a local variable).

All SDK Contribution checklist:

• The pull request does not introduce [breaking changes]
• CHANGELOG is updated for new features, bug fixes or other significant changes.
• I have read the contribution guidelines.

General Guidelines and Best Practices

• Title of the pull request is clear and informative.
• There are a small number of commits, each of which have an informative message. This means that previously merged commits do not appear in the history of the PR. For more information on cleaning up the commits in your PR, see this page.

Testing Guidelines

• Pull request includes test coverage for the included changes.

[Copilot]

Pull request overview

This PR adds comprehensive documentation for the options parameter in EnvironmentVariableLoader that shields secret environment variable values from appearing in test logs. The documentation explains how to use the EnvironmentVariableOptions class with the hide_secrets parameter and provides warnings about proper usage to avoid secret leaks.

Changes:

• Added a new section "Hide secret environment variables in test logs" to the devtools_testutils README
• Updated the EnvironmentVariableLoader usage documentation with details about the options parameter
• Added cross-reference link in the main tests documentation pointing to the new section

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 1 comment.

File	Description
eng/tools/azure-sdk-tools/devtools_testutils/README.md	Added comprehensive documentation for hiding secrets with EnvironmentVariableOptions, including usage examples and warnings
doc/dev/tests.md	Added cross-reference to the new secret hiding documentation