docs: update EPP protocol spec with streaming mode and health check requirements

[Neha-dot-Yadav]

What type of PR is this?

/kind documentation

What this PR does / why we need it:

This PR updates the Endpoint Picker Protocol (EPP) specification document to include two previously undocumented requirements:

1. Streaming Mode Support: Documents that the EPP MUST support streaming mode for inference requests and responses, enabling full-duplex communication for real-time AI inference workloads.

2. Health Checking: Documents the gRPC health checking protocol implementation, including:

   • Liveness checks (process health)
   • Readiness checks (service readiness)
   • External processor service checks
   • Leader election support for multi-replica deployments

These features are already implemented in the codebase (see cmd/epp/runner/health.go and pkg/epp/handlers/server.go) but were missing from the protocol specification document. This PR brings the documentation in sync with the actual implementation.

Which issue(s) this PR fixes:

Fixes #994

Does this PR introduce a user-facing change?:

Updated EPP protocol specification to document streaming mode and health checking requirements

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[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is NOT APPROVED

This pull-request has been approved by: Neha-dot-Yadav
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[k8s-ci-robot]

Hi @Neha-dot-Yadav. Thanks for your PR.

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[Neha-dot-Yadav]

@ahg-g @kfswain
Could you please review when you have a moment? This updates the EPP protocol spec documentation to match the implementation.

[ahg-g]

/assign @danehans
/ok-to-test