Fix dashboard underscore environment variable lookup

[hutiefang76]

Describe what this PR does / why we need it

The dashboard documentation uses underscore-style environment variables such as sentinel_dashboard_auth_password, but DashboardConfig only looked up the dotted key (sentinel.dashboard.auth.password).

This adds an underscore environment variable fallback for dotted config names, while keeping the existing exact environment variable lookup and system property fallback order unchanged.

Does this pull request fix one issue?

Fixes #3543

Describe how you did it

When reading a config key from the environment, DashboardConfig now:

1. checks the exact environment variable name first;
2. checks the dotted name converted to underscore form;
3. falls back to the existing system property lookup.

A unit test covers sentinel_dashboard_auth_password resolving through getAuthPassword().

Describe how to verify it

I verified the new behavior with DashboardConfigTest:

mvn -s /tmp/codex-empty-maven-settings.xml -pl sentinel-dashboard -am -DskipTests test-compile

CP="sentinel-dashboard/target/test-classes:sentinel-dashboard/target/classes:sentinel-core/target/classes:$HOME/.m2/repository/junit/junit/4.12/junit-4.12.jar:$HOME/.m2/repository/org/hamcrest/hamcrest-core/1.3/hamcrest-core-1.3.jar:$HOME/.m2/repository/com/github/stefanbirkner/system-rules/1.16.1/system-rules-1.16.1.jar:$HOME/.m2/repository/commons-lang/commons-lang/2.6/commons-lang-2.6.jar:$HOME/.m2/repository/org/springframework/spring-core/5.3.18/spring-core-5.3.18.jar"
java --add-opens java.base/java.util=ALL-UNNAMED -cp "$CP" org.junit.runner.JUnitCore com.alibaba.csp.sentinel.dashboard.config.DashboardConfigTest

Result:

OK (3 tests)

git diff --check also passed.

Special notes for reviews

The local Maven/Surefire run selected the JUnit Platform provider because spring-boot-starter-test brings in JUnit Jupiter, but this test class is JUnit 4 and no Vintage engine is present. I used JUnitCore directly so the existing JUnit 4 test class actually runs instead of reporting zero tests.

[CLAassistant]

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