feat(client): add request rate limiting

[gijzelaerr]

Summary

• add opt-in per-client request limiting at the outbound S7 PDU boundary
• support evenly spaced fixed-rate and burst-capable token-bucket algorithms
• support blocking, immediate rejection, and explicitly marked dropped-request behavior
• cover both synchronous and asynchronous clients
• document configuration and add deterministic limiter tests

A multi-variable PDU counts as one request, while chunked and multi-packet operations count each actual PDU. Model-specific defaults are intentionally not inferred: the limiter is disabled by default and users choose a rate appropriate for their PLC and workload.

Silent drops are unsafe because the client would wait for a response to a request that was never sent. Both raise and drop therefore surface S7RateLimitError; the latter sets dropped=True.

Validation

• uv run --frozen pre-commit run --all-files
• uv run --frozen pytest -q (1649 passed, 82 skipped)
• uv build

Fixes #628