refactor: return nil instead of "ok" from handlers

[fank]

Summary

• Change all handlers to return nil instead of "ok" for success
• formatDispatchResponse already converts nil to ["ok"]
• Avoids redundant ["ok", "ok"] responses

Test plan

• Build succeeds
• Verify extension returns ["ok"] for commands that previously returned ["ok", "ok"]

[github-actions]

Merging this branch will not change overall coverage

Impacted Packages	Coverage Δ	🤖
github.com/OCAP2/extension/v5/cmd/ocap_recorder	0.00% (ø)
github.com/OCAP2/extension/v5/internal/dispatcher	17.52% (ø)
github.com/OCAP2/extension/v5/internal/worker	1.77% (ø)

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Coverage by file

Changed files (no unit tests)

Changed File	Coverage Δ	Total	Covered	Missed	🤖
github.com/OCAP2/extension/v5/cmd/ocap_recorder/main.go	0.00% (ø)	0	0	0
github.com/OCAP2/extension/v5/internal/worker/dispatch.go	2.02% (ø)	2080	42	2038

Please note that the "Total", "Covered", and "Missed" counts above refer to code statements instead of lines of code. The value in brackets refers to the test coverage of that file in the old version of the code.

Changed unit test files

• github.com/OCAP2/extension/v5/internal/dispatcher/dispatcher_test.go
• github.com/OCAP2/extension/v5/internal/worker/dispatch_test.go