Add unit tests for methods in frontend API handler

[timl3136]

What changed? Add unit tests for RespondDecisionTaskCompleted in frontend api handler

Why? Improve test coverage

How did you test it? Unit tests

Potential risks

Release notes

Documentation Changes

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 71.17%. Comparing base (aad09de) to head (d85f496). Report is 1 commits behind head on master.

:exclamation: Current head d85f496 differs from pull request most recent head 4c0d1da

Please upload reports for the commit 4c0d1da to get more accurate results.

Additional details and impacted files

[see 13 files with indirect coverage changes](https://app.codecov.io/gh/uber/cadence/pull/6119/indirect-changes?src=pr&el=tree-more&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=uber) ------ [Continue to review full report in Codecov by Sentry](https://app.codecov.io/gh/uber/cadence/pull/6119?dropdown=coverage&src=pr&el=continue&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=uber). > **Legend** - [Click here to learn more](https://docs.codecov.io/docs/codecov-delta?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=uber) > `Δ = absolute (impact)`, `ø = not affected`, `? = missing data` > Powered by [Codecov](https://app.codecov.io/gh/uber/cadence/pull/6119?dropdown=coverage&src=pr&el=footer&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=uber). Last update [aad09de...4c0d1da](https://app.codecov.io/gh/uber/cadence/pull/6119?dropdown=coverage&src=pr&el=lastupdated&utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=uber). Read the [comment docs](https://docs.codecov.io/docs/pull-request-comments?utm_medium=referral&utm_source=github&utm_content=comment&utm_campaign=pr+comments&utm_term=uber).

[coveralls]

Pull Request Test Coverage Report for Build 018feab2-d9fc-47c6-b1ec-38eeb350d343

Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 545 unchanged lines in 15 files lost coverage.
• Overall coverage decreased (-0.3%) to 70.847%

---

Files with Coverage Reduction	New Missed Lines	%
service/history/queue/timer_queue_processor_base.go	1	77.66%
common/task/weighted_round_robin_task_scheduler.go	2	88.56%
service/frontend/api/handler.go	2	69.31%
service/history/task/transfer_active_task_executor.go	2	72.77%
common/task/fifo_task_scheduler.go	3	84.54%
common/persistence/statsComputer.go	3	98.21%
service/history/task/fetcher.go	3	86.6%
common/archiver/filestore/historyArchiver.go	4	80.95%
common/types/mapper/thrift/shared.go	4	98.31%
service/history/task/transfer_standby_task_executor.go	6	86.33%
<!--	Total:	545		-->

Totals
Change from base Build 018feaa3-8a65-4c9c-874c-0243b9141bed:	-0.3%
Covered Lines:	105521
Relevant Lines:	148942

---

💛 - Coveralls

[coveralls]

Pull Request Test Coverage Report for Build 018feecd-c54b-4228-8334-8ea702900f98

Details

• 0 of 0 changed or added relevant lines in 0 files are covered.
• 49 unchanged lines in 8 files lost coverage.
• Overall coverage decreased (-0.008%) to 71.292%

---

Files with Coverage Reduction	New Missed Lines	%
common/persistence/sql/sqlplugin/postgres/task.go	2	73.4%
common/util.go	2	91.84%
service/history/task/transfer_standby_task_executor.go	3	86.33%
service/history/task/task.go	3	84.81%
common/task/fifo_task_scheduler.go	4	82.47%
service/history/execution/cache.go	6	74.61%
common/persistence/sql/sqlplugin/postgres/db.go	9	65.0%
service/history/task/task_util.go	20	70.57%
<!--	Total:	49		-->

Totals
Change from base Build 018fee53-2f0a-42d7-9d39-e20b5cb03809:	-0.008%
Covered Lines:	106194
Relevant Lines:	148957

---

💛 - Coveralls