Fix name resolver bridge listener handling

[oliviamariacodes]

Fixes #12444

This PR addresses a bug in the NameResolver.Listener to NameResolver.Listener2 bridge affecting custom NameResolver implementations using Listener.

The bridge in NameResolver.start(Listener) at https://github.com/grpc/grpc-java/blob/master/api/src/main/java/io/grpc/NameResolver.java#L100 unconditionally calls getValue() on the StatusOr, throwing java.lang.IllegalStateException: No value present. when the result contains an error.

This was identified when upgrading from gRPC v1.63.3 to v1.75.0.

The bug occurs due to DnsNameResolver's error handling changes between versions:

• v1.63.3: Errors reported via Listener.onError() (https://github.com/grpc/grpc-java/blob/v1.63.x/core/src/main/java/io/grpc/internal/DnsNameResolver.java#L319)
• v1.75.0: Errors passed via Listener2.onResult2() with a ResolutionResult containing either addresses OR an error (https://github.com/grpc/grpc-java/blob/master/core/src/main/java/io/grpc/internal/DnsNameResolver.java#L322)

This PR updates the bridge to check whether ResolutionResult contains addresses or an error. It passes the error via onError and addresses via onAddresses.

Reproducing the Issue
The startOnOldListener_resolverReportsError test reproduces a similar issue. It creates a custom NameResolver that reports errors through the ResolutionResult like the DNSNameResolver in v1.75.0, passes an old Listener to resolver.start, which triggers the bridge code path.

Without the fix, the bridge calls getValue() on the error containing StatusOr, throwing IllegalStateException: No value present.

With the fix, the bridge checks hasValue() first and correctly routes to listener.onError() when appropriate. This ensures backward compatibility for Listener implementations when resolvers report errors via ResolutionResult.

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