[kalragauri]

InstantiatingGrpcChannelProvider wrapped DirectPath over Interconnect
channels in GoogleDefaultChannelCredentials, whose negotiator selects
ALTS for every xDS cluster it does not recognize as a CFE
("google_cfe_") cluster. The regional Interconnect clusters that RLS
returns for data operations are not CFE clusters, and ALTS is
unavailable outside Google Cloud, so those connections fail once the
service routes to them.

The provider now builds CompositeChannelCredentials from
TlsChannelCredentials and the call credentials when Interconnect is
active, so every subchannel negotiates TLS. The call credentials stay on
the channel credentials because those are the only credentials the RLS
control channel inherits from the parent channel. ALTS hard-bound tokens
are not requested on this path; they need an ALTS connection. Standard
DirectPath is unchanged.

DirectPath over Interconnect also requires credentials now, as standard
DirectPath does. Previously a channel built with NoCredentialsProvider
still used DirectPath, and GoogleDefaultChannelCredentials loaded
Application Default Credentials on its own, or failed every RPC with
UNAUTHENTICATED when none were available. Such a channel now falls
back to CloudPath.

The misconfiguration log names the missing credentials instead of
asking for ComputeEngineCredentials. That message is logged at
WARNING even off GCE, where Interconnect is meant to run; every other
message keeps its existing level, and a client that only inherits the
GOOGLE_CLOUD_ENABLE_DIRECT_PATH_XDS_OVER_INTERCONNECT environment
variable without enabling DirectPath logs nothing, as before.

The credential selection moves into
createDirectPathChannelCredentials so it can be tested without
dialing. Tests cover the builder option, the environment variable
(including case and whitespace), invalid and "false" values, null
credentials with and without an mTLS client certificate provider, ALTS
hard-bound tokens on both paths, the credentials the channel builder
receives, and the log messages and levels. In java-storage,
StorageOptionsBuilderTest now attaches the client's credentials before
expecting DirectPath over Interconnect, and checks that NoCredentials
falls back to CloudPath; the setAttemptDirectPathXdsOverInterconnect
Javadoc documents the credentials and TLS requirements.

[gemini-code-assist]

Code Review

This pull request introduces support for DirectPath xDS over Interconnect off Google Compute Engine (GCE) by negotiating TLS instead of ALTS and requiring credentials. It updates credential compatibility checks, refactors channel credential creation, improves misconfiguration logging, and adds comprehensive unit tests. The review feedback recommends returning credentials != null in isCredentialDirectPathCompatible() to prevent a potential NullPointerException when credentials are null, and ensuring that CompositeChannelCredentials and TlsChannelCredentials are properly imported to avoid compilation errors.