feat: implement rateLimiterNode with fixed and sliding window strategies

[DheerajShrivastav]

feat: implement rateLimiterNode — fixed & sliding window rate limiting

Closes #11

What this adds

A rate_limiter primitive node that queues a list of items through a rate-limited pipeline, preventing downstream API calls from exceeding configured limits. Supports two strategies:

• Fixed window — processes items in batches of requestsPerWindow, sleeping windowMs between each batch
• Sliding window — tracks per-request timestamps; each item waits only until the oldest in-window request expires

Input schema

{
  items: unknown[]          // min 1 item — the payloads to rate-limit through
  requestsPerWindow: number // 1–10000
  windowMs: number          // 100–3600000 ms
  strategy: 'fixed' | 'sliding'
}

Note: items is not in the original spec but is required for the node to have anything to process. Without it the node would have no purpose as a standalone primitive.

Output schema

{
  processedItems: unknown[]   // all items in original order
  totalItems: number
  totalDurationMs: number     // actual wall-clock time (fake-time in tests)
  strategy: string
  windowMs: number
  requestsPerWindow: number
}

Design decisions

Decision	Reason
category: 'logic'	The spec lists "Primitives" but no such category exists in the codebase; 'logic' matches all comparable nodes (delayNode, conditionalNode)
supportsCancel: false	Consistent with all other logic nodes; cancellation would require a significantly different executor pattern
supportsRerun: true	Deterministic output for the same input — safe to rerun
Runtime guard for requestsPerWindow < 1	Belt-and-suspenders beyond Zod min(1); prevents an infinite loop if the executor is called directly

Tests — 28 cases covering

• Metadata (type, category, capabilities)
• Input schema validation (all boundary conditions)
• Fixed window: ordering, metadata, exact-fit batches, timing assertion (totalDurationMs >= windowMs)
• Sliding window: ordering, strategy echo, single-item no-wait, timing assertion
• Output schema: valid shape, missing fields, negative duration rejection
• Error handling: schema-level requestsPerWindow: 0 rejection + executor-level runtime guard