[code-coverage] Add tests for HandleDecisionTaskStarted

[ketsiambaku]

What changed?
Add tests for HandleDecisionTaskStarted in service/history/decision

Why?
Improve code coverage

How did you test it?
unit tests

Potential risks
n/a

Release notes

Documentation Changes

[codecov]

Codecov Report

Merging #5906 (8917670) into master (5b27da0) will increase coverage by 0.06%.
The diff coverage is n/a.

❗ Current head 8917670 differs from pull request most recent head 05ac24a. Consider uploading reports for the commit 05ac24a to get more accurate results

Additional details and impacted files

see 10 files with indirect coverage changes

---

Continue to review full report in Codecov by Sentry.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 5b27da0...05ac24a. Read the comment docs.

[coveralls]

Pull Request Test Coverage Report for Build 018ed314-d8e2-4984-9f8e-529c7d7c43b3

Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 29 unchanged lines in 7 files lost coverage.
• Overall coverage increased (+0.04%) to 67.389%

---

Files with Coverage Reduction	New Missed Lines	%
common/task/weighted_round_robin_task_scheduler.go	2	88.56%
common/persistence/sql/sqlplugin/mysql/task.go	2	73.68%
service/matching/taskListManager.go	2	80.2%
common/membership/hashring.go	2	84.69%
common/persistence/sql/sqlplugin/mysql/db.go	2	79.49%
service/history/task/transfer_standby_task_executor.go	4	86.19%
service/history/execution/mutable_state_task_refresher.go	15	68.67%

Totals
Change from base Build 018ed015-a20c-4f3d-9b31-3918611b3162:	0.04%
Covered Lines:	98520
Relevant Lines:	146197

---

💛 - Coveralls