Add rate_limiting_active field to TaskQueueStats (#818)

## What

Adds `bool rate_limiting_active` to `TaskQueueStats` proto message.

## Why

Auto-scaling systems need to know when backlog growth is caused by rate
limiting rather than insufficient worker capacity. When the whole-queue
rate limit is blocking dispatches, scaling up workers doesn't help — the
bottleneck is the rate limit, not worker count.

[Server PR](temporalio/temporal#10944)

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Co-authored-by: Claude Opus 4.6 <noreply@anthropic.com>

Author: rkannan82