refactor(agent-runtime): extract helpers from run-agent-step.ts (Commit 2.3)

Extracts 6 helper functions from run-agent-step.ts into agent-step-helpers.ts:
- initializeAgentRun: Resolves agent template, starts run, builds system prompt and tools
- buildInitialMessages: Constructs initial message history
- buildToolDefinitions: Converts ToolSet to serializable format for token counting
- prepareStepContext: Prepares context for agent step including token counting
- handleOutputSchemaRetry: Handles missing output schema validation
- Error utilities: extractErrorMessage, isPaymentRequiredError, getErrorStatusCode

Includes comprehensive unit tests (35 tests) with section headers for readability.

Fixes model parameter in callTokenCountAPI to ensure correct tokenizer is used.

Author: brandonkachen