feat(inngest): expand execution request model

[darwin67]

Summary

Implements plan 002 by expanding the execution request model so shared code can carry the spec-required call context for payload recovery and planned-step execution.

Changes:

• Parse and thread disable_immediate_execution, use_api, stack, and qi_id through an internal execution context.
• Capture inbound stepId in Ktor and Spring adapters and pass it into shared execution.
• Expand protocol fixtures and tests for richer execution request payloads while preserving minimal-payload compatibility.
• Preserve the Java four-argument ExecutionContext constructor with an interop test.
• Mark plan 002 complete.

Checklist

• Update documentation
• Added unit/integration tests

Related

• Plan: docs/plans/002-expand-execution-request-model.org