feat: support editable class labels in question classifier

[Blackoutta]

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Summary

Fixes #35429

Refs #33806
Supersedes #34075
Companion Graphon branch: langgenius/graphon:blackoutta/question-classifier-labels-graphon
Companion Graphon PR URL: https://github.com/langgenius/graphon/pull/new/blackoutta/question-classifier-labels-graphon

This recreates the Dify-side portion of editable Question Classifier labels after the workflow runtime moved into graphon.

The UI now treats Question Classifier branch data as three separate concepts:

• id: stable branch identifier for routing/handles
• name: classifier-facing category description used for prompt logic and preserved as class_name
• label: user-facing branch title shown in the editor, canvas, and preview

Included changes:

• adds inline editing for Question Classifier branch labels
• preserves existing class_name semantics for downstream descriptive output
• exposes class_label in Question Classifier output configuration
• updates node and preview rendering to display label with sensible CLASS N fallbacks
• adds frontend tests covering editing, rendering, and output behavior
• includes panel-specific regression fixes required by the Question Classifier editing flow

This branch was validated locally against an editable Graphon checkout wired through uv.

From Codex

Screenshots

Before	After
Branch titles were fixed to CLASS N in the UI.	Branch titles can be edited independently from the classifier description text.

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

[autofix-ci]

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[Blackoutta]

Heads-up on merge order for this feature: this PR depends on the runtime/schema changes landing in Graphon first.

Even though the Dify-side UI is ready, we should wait until the companion Graphon PR is merged and a new Graphon version is released before merging this PR. After that, this PR should be updated to bump graphon in api/pyproject.toml to the released version and rerun CI against that published package.

Without that release order, users can see/edit the new classifier labels in the UI, but workflow execution may still run against an older Graphon runtime that does not fully support the new label / class_label behavior.

So the intended sequence is:

1. Merge the Graphon PR
2. Cut a new Graphon release
3. Update this Dify PR to the released Graphon version
4. Merge this PR

Companion Graphon PR: https://github.com/langgenius/graphon/pull/new/blackoutta/question-classifier-labels-graphon

[RiskeyL]

docs update: langgenius/dify-docs@5da0923#diff-7f97af6b4092b78f2fa0ffd6c2d5ef65ae7c547561085ebf06acc057d6abcc34