Add per-agent concurrency limit for connector-triggered agents

## Summary
Customer requests a per-agent concurrency cap for connector-triggered agents, similar to tunable HTTP trigger concurrency, to reduce overlapping runs during bursts of connector events.

## Customer excerpt
> Is there a way to cap the maximum concurrent invocations for a single connector-triggered agent, similar to how HTTP trigger concurrency can be tuned? We have a scenario where bursts of connector events cause too many overlapping runs, and a per-agent concurrency cap would help us control cost and avoid downstream rate limits.

---
Auto-triaged from the serverless-agents-crew@microsoft.com mailbox.
Reporter: Laveesh.Rohra@microsoft.com

Start by locating the connector-triggered agent invocation and the existing tunable HTTP trigger concurrency handling. Define how a per-agent cap is configured and enforced during connector event bursts, then verify that overlapping runs are limited as requested without changing unrelated triggers.

Written by the indexing model from the issue text.