SLO Aware Routing PD Disaggregation Support

[RishabhSaini]

Add pod_type support for prefill-decode (PD) disaggregated serving:

• Added pod_type categorical feature (''=monolithic, 'prefill', 'decode') to both TTFT and TPOT prediction models
• Implemented pod_type_cat encoding in feature preparation with proper categorical handling
• Updated Bayesian Ridge models to one-hot encode pod_type (can't handle categoricals directly)
• Modified XGBoost monotone constraints from 5 -> 6 features, with no constraint on pod_type
• Added pod_type field to API models (PredictionRequest, TrainingEntry) in Python and Go
• Ensured backward compatibility by defaulting to '' (monolithic) when pod_type is missing

Resolves: #1923

Links to related PRs:
llm-d/llm-d#442
llm-d/llm-d-inference-scheduler#511

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