test: add missing tests to achieve 100% coverage

[Copilot]

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2. Ensure there is an associated issue and you have been assigned to it

Summary

This PR addresses the missing line coverage across several backend and frontend modules as highlighted in the issue. The goal was to reach 100% coverage on specific files to strengthen the testing baseline without altering the existing production logic.

Backend (api/) additions:

• api/core/llm_generator/llm_generator.py: Added coverage for JSON repair fallbacks in _parse_string_list, context building failures in generate_workflow_instruction_suggestions, and edge cases in classify_workflow_mode.
• api/controllers/console/app/generator.py: Added exhaustive testing of missing payload argument permutations and validation exception handlers (e.g. ProviderTokenNotInitError, QuotaExceededError) for RuleGenerateApi, RuleCodeGenerateApi, StructuredOutputGenerateApi, and InstructionGenerateApi.
• api/core/workflow/generator/runner.py: Added test cases for Kahn's cycle edge detection errors, empty iteration container error handling, and unresolved graph node reference reporting (UNKNOWN_NODE_REFERENCE, UNKNOWN_TOOL, INVALID_CONTAINER).

Frontend (web/) additions:

• web/service/base.ts: Expanded handleStream mock reader coverage to deal with malformed JSON buffer objects, errors on 400 statuses, and reasoning chunk dispatches. Verified standard HTTP export methods (get, post, del etc).
• web/service/debug.ts: Verified all exported functional signatures including sendCompletionMessage, fetchSuggestedQuestions, fetchConversationMessages, and generateWorkflowStream with respective parameter/callback wiring logic.

Screenshots

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Checklist

• This change requires a documentation update, included: Dify Document
• I understand that this PR may be closed in case there was no previous discussion or issues. (This doesn't apply to typos!)
• I've added a test for each change that was introduced, and I tried as much as possible to make a single atomic change.
• I've updated the documentation accordingly.
• I ran make lint && make type-check (backend) and cd web && pnpm exec vp staged (frontend) to appease the lint gods