[pluggable parser] EPP should fail at startup on InferencePool appProtocol and gRPC parser mismatch

[zetxqx]

Currently, EPP does not strictly validate the alignment between the InferencePool configuration and the internal parser settings during the startup sequence.

Since EPP watches the InferencePool, it has direct visibility into the underlying appProtocol. If an operator configures a gRPC parser (via EndpointPickerConfig.parser) but the InferencePool defines a conflicting or incompatible appProtocol, the system may start in an inconsistent state, leading to difficult-to-debug runtime errors.

Proposed Behavior:

EPP should perform a validation check during its initialization phase:

• Inspect the appProtocol defined in the InferencePool. Compare this protocol against the configured parser (e.g., ensuring a gRPC parser is only used when the protocol is gRPC-compatible).
• Fail Fast: If a mismatch is detected, EPP should log a clear error message describing the conflict and terminate the startup process.

[zetxqx]

/assign