feat: agent, knowledge base, and tool workflows support default model…

[shenkaibo]

… configuration and switching

What this PR does / why we need it?

Adds support for configuring and switching default models across agent, knowledge base, and tool workflows. Users can now set category-level default models (LLM, embedding, reranker, STT/TTS, etc.) from the workflow dropdown menu, and any node can run in "default model" mode to automatically use the configured default for its category. This removes the need to pick a model on every node and makes large-scale model switching much easier.

Highlights:

• New "default model settings" panel from the workflow dropdown menu, styled and positioned consistently with the "Add component" dialog.
• "Apply to all nodes" action to bulk-set default model usage across the workflow.
• Node-level "default model" mode that displays the resolved default model name (grayed out, read-only).
• Model resource mapping includes custom models on base nodes and distinguishes between "not configured" and "unavailable" states.
• Publish-time validation (frontend + backend) blocks publishing when a node in "default model" mode has no default model configured for its category, with i18n-friendly messages referencing the actual node name.
• Default model panel changes are synced to the workflow detail in real time and used by the latest publish validation.

Summary of your change

Backend

• Added default_model_setting to application / knowledge workflow / tool workflow models with DB migrations (0015, 0011, 0008).
• Adapted workflow node implementations (ai_chat, reranker, image/text/video, STT/TTS, intent, parameter extraction, question, knowledge, tool workflow lib, etc.) to resolve category default models.
• Updated serializers to accept/return default model settings.
• Aligned publish validation between backend and frontend.

Frontend

• New default-model-setting panel component and DefaultModelDisplay component.
• Base-node and per-node UI support "default model" mode with a grayed read-only model name.
• Publish-time pre-validation intercepts nodes that select "default model" when the category default is not configured.
• i18n updates for zh-CN, en-US, and zh-Hant.

Please indicate you've done the following:

• Made sure tests are passing and test coverage is added if needed.
• Made sure commit message follow the rule of Conventional Commits specification.
• Considered the docs impact and opened a new docs issue or PR with docs changes if needed.