ci: add check for 'do not merge' in PR titles

[devin-ai-integration]

Add check for 'do not merge' in PR titles

This PR adds a check to prevent PRs with 'do not merge' or 'do-not-merge' in the title from merging. The check is case-insensitive and runs after the semantic PR title check.

The check will fail if a PR title contains either "do not merge" or "do-not-merge" anywhere in the title, regardless of casing or surrounding characters.

Link to Devin run: https://app.devin.ai/sessions/bde79d1047584bf09327a9c5f38873c8

Requested by: Aaron (Aaron ("AJ") Steers (@aaronsteers))

[devin-ai-integration]

🤖 Devin AI Engineer

Original prompt from Aaron:

Received message in Slack channel #dev-devin-ai:

@Devin - I noticed some CDK PRs are merging with "(do not merge)" in the title. I'd like to prevent this in one of these two methods:
1. Add logic into the semantic PR title CI check, if it is supported to do so.
2. Otherwise, do our own check immediately after to that step running - raising an error there if "do not merge" exists in the PR title (in any casing).
Don't change the conditions of the CI job running, only adding this new failure condition when it is running.

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[Aaron ("AJ") Steers (aaronsteers)]

Merging with failed test. The only failed test is the one that proves the behavior we wanted. 🚀

https://github.com/airbytehq/airbyte-python-cdk/actions/runs/13662746310/job/38197414923?pr=383