feat: add pdrole package for P/D role discovery and detection

[ev-shindin]

Summary

• Add internal/utils/pdrole package that discovers P/D disaggregation configuration from the EPP's EndpointPickerConfig and detects each deployment's P/D role from pod template labels
• Discovery is per-pool (accepts *pool.EndpointPool), enabling correct behavior when multiple EPPs exist with different P/D label settings
• Returns PDDiscoveryResult with Disaggregated flag so callers can distinguish "no P/D plugins" (all pods serve both roles) from "P/D plugins found, use label config"

Design decisions

Label-only detection (no deployment name fallback).
The EPP's filter plugins (prefill-filter, decode-filter, by-label from llm-d-inference-scheduler) route traffic based solely on pod labels. A deployment named llama-prefill without a llm-d.ai/role label would still receive decode traffic from EPP (decode-filter has allowsNoLabel=true). Name-based guessing would misclassify it.

BothValues from intersection, not hardcoded.
For by-label custom plugins, values appearing in both prefill and decode profiles' validValues are classified as BothValues. This ensures GetDeploymentPDRole returns RoleBoth (not RolePrefill or RoleDecode) for pods that pass both filters.

Aligned with EPP's actual filter semantics:

EPP filter	Label	Valid values	allowsNoLabel
prefill-filter	llm-d.ai/role	"prefill"	false
decode-filter	llm-d.ai/role	"decode", "both"	true

Package structure

File	Purpose
types.go	PDRole, PDRoleLabelConfig, PDDiscoveryResult, constants
detect.go	GetDeploymentPDRole — label-based role detection
discover.go	DiscoverPDRoleLabelConfig — EPP config discovery chain
*_test.go	43 Ginkgo specs

Discovery chain

1. Pool -> EPP service name/namespace
2. Service selector -> EPP deployment
3. Deployment volumes -> mounted ConfigMap
4. ConfigMap data -> parse EndpointPickerConfig (YAML/JSON)
5. Plugins -> detect prefill-filter/decode-filter (well-known label) or by-label (custom label from parameters)
6. Any step fails -> Disaggregated=false, default config

Intended caller pattern (not wired yet)

pool, _ := datastore.PoolGetFromLabels(deploy.Spec.Template.Labels)
result := pdrole.DiscoverPDRoleLabelConfig(ctx, k8sClient, pool)
if !result.Disaggregated {
    // no P/D plugins — all deployments serve both roles
    role = pdrole.RoleBoth
} else {
    role = pdrole.GetDeploymentPDRole(deploy, result.LabelConfig)
}

Test plan

• go build ./internal/utils/pdrole/... — clean
• go vet ./internal/utils/pdrole/... — clean
• go test ./internal/utils/pdrole/... -v -count=1 — 43/43 specs pass
• No callers yet (infrastructure-only PR), wiring comes in follow-up