ci: run main.yml checks on merge_group (prerequisite for a merge queue) (#423)

ci: run main.yml checks on merge_group (prerequisite for enabling a merge queue)

Adds a `merge_group` trigger so lint / unit-test / e2e-test / coverage run on the
merge queue's transient ref. This MUST land before a merge queue is enabled on
`main`: the queue gates each merge group on these required checks, and without
the trigger they'd never run, stalling every queued PR. Mirrors the Python
connector (code-coverage.yml runs on merge_group for the same reason).

No job-level event guards, so all four required-check jobs run on merge_group
exactly as they do on pull_request.

Co-authored-by: Isaac

Signed-off-by: Madhavendra Rathore <madhavendra.rathore@databricks.com>

Author: msrathore-db

.github/workflows/main.yml

@@ -7,6 +7,11 @@ on:
   pull_request:
     branches:
       - main
+  # Run the same required checks (lint / unit-test / e2e-test / coverage) on the
+  # merge queue's transient ref. Required so a merge queue can be enabled on
+  # `main`: the queue waits on these checks for each merge group, and without a
+  # `merge_group` trigger they would never run and every queued PR would stall.
+  merge_group:
 
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