fix(emqx_connector): report errors in on_start handler

[sstrigler]

Fixes EMQX-10174

This PR fixes the HTTP connector for webhooks and IoTDB. There will be a follow up for kafka.

Summary

🤖 Generated by Copilot at 8804560

This pull request improves the test code and the error handling of the emqx_bridge and emqx_connector applications. It refactors the emqx_bridge_testlib and emqx_bridge_webhook_SUITE modules to make them more readable and robust. It also fixes a bug in the emqx_connector_http module that caused the connector to report a wrong status when the HTTP connection failed.

PR Checklist

Please convert it to a draft if any of the following conditions are not met. Reviewers may skip over until all the items are checked:

• Added tests for the changes
• Changed lines covered in coverage report
• Change log has been added to changes/{ce,ee}/(feat|perf|fix)-<PR-id>.en.md files
• For internal contributor: there is a jira ticket to track this change
• If there should be document changes, a PR to emqx-docs.git is sent, or a jira ticket is created to follow up
• Schema changes are backward compatible

[coveralls]

Pull Request Test Coverage Report for Build 5264714003

• 10 of 15 (66.67%) changed or added relevant lines in 1 file are covered.
• 21 unchanged lines in 6 files lost coverage.
• Overall coverage increased (+0.08%) to 81.968%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
apps/emqx_connector/src/emqx_connector_http.erl	10	15	66.67%

Files with Coverage Reduction	New Missed Lines	%
apps/emqx/src/emqx_broker.erl	1	88.2%
apps/emqx/src/emqx_channel.erl	1	87.86%
apps/emqx/src/emqx_logger.erl	1	80.49%
apps/emqx/src/emqx_message.erl	1	94.83%
apps/emqx/src/emqx_flapping.erl	3	85.48%
apps/emqx/src/emqx_reason_codes.erl	14	87.5%

Totals
Change from base Build 5244853575:	0.08%
Covered Lines:	29737
Relevant Lines:	36279

---

💛 - Coveralls