fix(workflow): reject invalid case expressions and missing action conf

[Arjen10]

Description

check_schema inspected the wrong variable after expr.new, so illegal case values passed validation and matched every request at runtime. Missing action conf also indexed a nil value and 500'd the Admin API.

Which issue(s) this PR fixes:

Fixes #13910

Checklist

• I have explained the need for this PR and the problem it solves
• I have explained the changes or the new features added to this PR
• I have added tests corresponding to this change
• I have updated the documentation to reflect this change
• I have verified that this change is backward compatible (If not, please discuss on the APISIX mailing list first)

[janiussyafiq]

Thanks for fixing the CI lint, however a few points need to be addressed:

1. CI lint. apisix/plugins/workflow.lua:144 uses the global type without local type = type, so lj-releng errors. You can run CI locally first to check if it passes.
2. Empty actions: [] still crashes at runtime. Schema has minItems on each action but not on actions itself, so the validation loop is a no-op. Admin API returns passed, then every request on that rule gets 500 at workflow.lua:174 (rule.actions[1] is nil).
3. Non-string action name 500s. workflow.lua:141 concatenates action[1], so [true], [{}], [null] crash with attempt to concatenate.
4. Test gap. New tests only unit-call check_schema. No Admin-API-level error_code: 400 assertion for the issue's repro, no empty-actions case. t/plugin/workflow2.t:316 has the pattern to reuse.

Design:
One schema change beats the guard by declaring actions.items as [string, object] with minItems: 2. WDYT?

[Arjen10]

Thanks for fixing the CI lint, however a few points need to be addressed:

1. CI lint. apisix/plugins/workflow.lua:144 uses the global type without local type = type, so lj-releng errors. You can run CI locally first to check if it passes.
2. Empty actions: [] still crashes at runtime. Schema has minItems on each action but not on actions itself, so the validation loop is a no-op. Admin API returns passed, then every request on that rule gets 500 at workflow.lua:174 (rule.actions[1] is nil).
3. Non-string action name 500s. workflow.lua:141 concatenates action[1], so [true], [{}], [null] crash with attempt to concatenate.
4. Test gap. New tests only unit-call check_schema. No Admin-API-level error_code: 400 assertion for the issue's repro, no empty-actions case. t/plugin/workflow2.t:316 has the pattern to reuse.

Design: One schema change beats the guard by declaring actions.items as [string, object] with minItems: 2. WDYT?

Thank you for the code review. i went throuth the workflow plugin doc again. i think your solution is better. i will fix it

[Arjen10]

merge #13921

[membphis]

LGTM