[Refactor/Chore] Add preflight workflow validation and actionable diagnostics for missing variable references / unreachable paths

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### Description

**Description**
When an Advanced Workflow contains graph or variable-dependency mistakes, the current UX/backend feedback is too opaque. In my case, the chat stayed on `cacheing variables`, SSE returned `internal_server_error`, and the frontend then requested suggested questions for an answer placeholder and got `404`. The actual root causes were workflow structure and variable-reference issues, such as missing variables from parallel or conditional branches (for example in my case `#1440.text#` and `#1811.text#`).

I am proposing a refactor/chore to improve workflow validation and runtime diagnostics so authors can identify these issues before or during execution, instead of only seeing generic server errors.

### Motivation

Now it is hard to tell whether a failure is caused by:

* an unreachable path in the workflow graph,
* a missing edge after refactoring,
* a node referencing output from a conditional branch that did not execute,
* or a node depending on parallel branch outputs that were never produced.

A better developer experience would include:

* preflight validation before publish/run,
* node-level error messages,
* variable dependency checks for conditional/parallel paths,
* and a clearer frontend failure state instead of hanging on `cacheing variables`.

### Additional Context

Suggested checks:

1. Warn if a node references another node’s output that is not guaranteed on all paths.
2. Warn if a downstream node depends on multiple parallel branches without safe fallback/join semantics.
3. Show node id + variable path in runtime errors.
4. Avoid follow-up frontend calls like suggested questions when the answer was never successfully created.

This would help workflow authors debug issues much faster and reduce generic `internal_server_error` cases that are actually workflow-definition problems. The failures I observed were reproducible in a real workflow + batch test, including compare-path missing variables and optimization-path missing variables.