Support event-driven PR comments for newly captured errors (including open and draft PRs)

### Problem Statement

Teams need a way to surface newly captured Sentry errors directly within relevant pull requests to improve visibility during testing and review. Today, PR comments are limited to existing unresolved issues on open PRs or retrospective suspect commit comments on merged PRs, and draft PRs are not supported. This makes it difficult to proactively alert developers when new errors are introduced during manual or sandbox testing, especially when Slack notifications are missed or noisy. Enabling configurable, event-driven comments on active (open/draft) PRs would provide more targeted, actionable feedback during the development lifecycle.

### Solution Brainstorm

_No response_

### Product Area

Unknown

The issue names no files, tests, or entry points. Start by tracing the existing pull-request comment integration and error-capture event handling, then identify the configuration and test locations before defining the behavior for newly captured errors on open and draft pull requests.

Written by the indexing model from the issue text.