refactor: extract payload builders and tracing into reusable modules

[Chibionos]

Summary

• Extracts evaluation reporting logic into dedicated modules for better code organization, reusability, and separation of concerns
• Adds EvalTracingManager class that encapsulates OpenTelemetry tracing logic for evaluation runs
• Adds _payload_builders package with abstract base class and concrete implementations for coded and legacy evaluations

Why This Refactoring Was Needed

The StudioWebProgressReporter class had grown to over 1200 lines with significant code duplication between coded and legacy evaluation handling. This refactoring:

1. Improves maintainability: Separates concerns into focused modules - tracing logic in _eval_tracing.py and payload building in _payload_builders/

2. Enables reusability: The new abstractions can be used independently for:

   • Building evaluation payloads for different backends
   • Managing OpenTelemetry tracing for evaluation runs
   • Extracting usage metrics from spans

3. Reduces duplication: Shared utilities like GUID conversion, usage extraction, and completion metrics building are now in a single base class

4. Facilitates testing: Smaller, focused classes are easier to unit test in isolation

New Modules

_eval_tracing.py

• EvalTracingManager: Manages OpenTelemetry tracing for evaluation runs including parent trace creation, eval run traces, and evaluator span management

_payload_builders/

• BasePayloadBuilder: Abstract base class with shared utilities for GUID conversion, usage extraction from spans, completion metrics, and request spec building
• CodedPayloadBuilder: Handles coded agent evaluation payloads with string IDs and /coded/ endpoint suffix
• LegacyPayloadBuilder: Handles legacy (low-code) agent payloads with GUID conversion and assertionRuns format

Test plan

• Verify lint checks pass (uv run just lint)
• Verify type checks pass (uv run mypy src/uipath/_cli/_evals/)
• Verify build succeeds (uv run just build)
• Run evaluation tests to ensure no regression

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