Refactor: extract shared step execution logic to reduce code duplication

[Copilot]

• Analyze codebase for refactoring opportunities
• Create shared helper functions for step execution logic
  • Added getFunctionType() to determine function arity type
  • Added executeStepFn() to handle step function execution
• Move shared utilities to shims/shared.ts to avoid circular dependency
• Refactor shims/deno.ts to use shared helpers
• Refactor shims/node.ts to use shared helpers
• Refactor shims/bun.ts to use shared helpers
• Fix circular dependency issue causing CI test failures
• Verify CI passes

Original prompt

Look for refactors cleaning up the code and reducing code mass. Keep it lean, remember to not add dependencies.

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[Hexagon]

@copilot review ci test failures

[Copilot]

@copilot review ci test failures

CI failures were caused by a circular dependency - the shims imported runtime functions from mod.ts, which itself has top-level await importing the shims. Fixed by moving shared utilities (getFunctionType, executeStepFn) to a new shims/shared.ts file. (a16cac2)