refactor(api): Separate SegmentType for Integer/Float to Enable Pydantic Serialization (#22025)

refactor(api): Separate SegmentType for Integer/Float to Enable Pydantic Serialization (#22025)

This PR addresses serialization issues in the VariablePool model by separating the `value_type` tags for `IntegerSegment`/`FloatSegment` and `IntegerVariable`/`FloatVariable`. Previously, both Integer and Float types shared the same `SegmentType.NUMBER` tag, causing conflicts during serialization.

Key changes:
- Introduce distinct `value_type` tags for Integer and Float segments/variables
- Add `VariableUnion` and `SegmentUnion` types for proper type discrimination
- Leverage Pydantic's discriminated union feature for seamless serialization/deserialization
- Enable accurate serialization of data structures containing these types

Closes #22024.

api/core/prompt/advanced_prompt_transform.py

@@ -158,7 +158,7 @@ class AdvancedPromptTransform(PromptTransform):
 
             if prompt_item.edition_type == "basic" or not prompt_item.edition_type:
                 if self.with_variable_tmpl:
-                    vp = VariablePool()
+                    vp = VariablePool.empty()
                     for k, v in inputs.items():
                         if k.startswith("#"):
                             vp.add(k[1:-1].split("."), v)