Fix GraalVM native image compatibility for AzureIdentityEnvVars

[g2vinay]

Resolves #47106

Replace reflection-dependent AzureIdentityEnvVars enum usage with direct string literal to avoid NullPointerException in GraalVM native images.

The ExpandableStringEnum base class uses reflection for static field initialization, which doesn't work in native images without explicit configuration. Using the string literal 'AZURE_TOKEN_CREDENTIALS' directly eliminates the reflection dependency while maintaining identical runtime behavior.

Fixes NullPointerException in IdentityClientOptions.loadFromConfiguration when running in GraalVM native builds (e.g., Quarkus applications).

[Copilot]

Pull request overview

This PR addresses a GraalVM native-image compatibility issue in azure-identity by removing a reflection-dependent reference to AzureIdentityEnvVars when reading the AZURE_TOKEN_CREDENTIALS configuration value, preventing a NullPointerException in native builds.

Changes:

• Removed AzureIdentityEnvVars usage from IdentityClientOptions.loadFromConfiguration and replaced it with the "AZURE_TOKEN_CREDENTIALS" string key.
• Added a changelog entry documenting the GraalVM native-image NullPointerException fix.

Reviewed changes

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

File	Description
sdk/identity/azure-identity/src/main/java/com/azure/identity/implementation/IdentityClientOptions.java	Avoids referencing AzureIdentityEnvVars during configuration loading to prevent GraalVM native-image reflection/static-init issues.
sdk/identity/azure-identity/CHANGELOG.md	Documents the GraalVM native-image NullPointerException fix in the unreleased section.

[anannya03]

I've a basic question- As per my understanding, when we reference the static fields in AzureIdentityEnvVars, it calls the fromString() method which inherently uses reflection. In a Native build like GraalVM, metadata isn't stored by default, so the reflections lookup fails and returns a null.

If a user running a native image tries to execute DAC->
DefaultAzureCredential cred = new DefaultAzureCredentialBuilder.requireEnvVar(AzureIdentityEnvVars.AZURE_TENANT_ID).build();,
wouldn't they encounter the same issue with static field being null?