fix:[FL-172][高程数据管理页面上传文件提交表单报错]

修复高程文件上传接口的422错误。

问题原因：
ElevationFileRecordCreateRequest schema 的 mount_code 字段设置了 min_length=2 约束，
当前端传递空字符串时，Pydantic 验证失败导致 422 Unprocessable Entity 错误。

解决方案：
1. 添加 field_validator，将空字符串转换为 None
2. 应用到以下 Request schemas 的可选字符串字段：
   - ElevationFileRecordCreateRequest: source, mount_code, notes
   - ElevationFileRecordUpdateRequest: source, notes
   - ElevationDatasetCreateRequest: source, mount_code, file_name, notes
   - ElevationDatasetUpdateRequest: name, source, notes

这样可以确保：
- 空字符串被规范化为 None
- 保持向后兼容性
- 后端逻辑可以正确使用默认 mount_code（第一个可用挂载点）

Co-authored-by: multica-agent <github@multica.ai>

api/app/schemas/elevation.py

@@ -3,7 +3,7 @@ from __future__ import annotations
 from datetime import datetime
 from typing import Any, Literal
 
-from pydantic import BaseModel, Field
+from pydantic import BaseModel, Field, field_validator
 
 
 ElevationDatasetStatus = Literal["active", "disabled"]
@@ -62,6 +62,13 @@ class ElevationFileRecordCreateRequest(BaseModel):
     notes: str | None = Field(default=None, max_length=2000)
     trigger_analysis: bool = Field(default=True)
 
+    @field_validator("source", "mount_code", "notes", mode="before")
+    @classmethod
+    def empty_str_to_none(cls, v: Any) -> Any:
+        if isinstance(v, str) and v.strip() == "":
+            return None
+        return v
+
 
 class ElevationFileRecordUpdateRequest(BaseModel):
     source: str | None = Field(default=None, max_length=512)
@@ -69,6 +76,13 @@ class ElevationFileRecordUpdateRequest(BaseModel):
     status: ElevationDatasetStatus | None = None
     notes: str | None = Field(default=None, max_length=2000)
 
+    @field_validator("source", "notes", mode="before")
+    @classmethod
+    def empty_str_to_none(cls, v: Any) -> Any:
+        if isinstance(v, str) and v.strip() == "":
+            return None
+        return v
+
 
 class ElevationFileRecordAnalyzeResponse(BaseModel):
     record: ElevationFileRecordSummary
@@ -165,6 +179,13 @@ class ElevationDatasetCreateRequest(BaseModel):
     resolution_m: float | None = Field(default=None, gt=0)
     notes: str | None = Field(default=None, max_length=2000)
 
+    @field_validator("source", "mount_code", "file_name", "notes", mode="before")
+    @classmethod
+    def empty_str_to_none(cls, v: Any) -> Any:
+        if isinstance(v, str) and v.strip() == "":
+            return None
+        return v
+
 
 class ElevationDatasetUpdateRequest(BaseModel):
     name: str | None = Field(default=None, min_length=2, max_length=255)
@@ -173,6 +194,13 @@ class ElevationDatasetUpdateRequest(BaseModel):
     status: ElevationDatasetStatus | None = None
     notes: str | None = Field(default=None, max_length=2000)
 
+    @field_validator("name", "source", "notes", mode="before")
+    @classmethod
+    def empty_str_to_none(cls, v: Any) -> Any:
+        if isinstance(v, str) and v.strip() == "":
+            return None
+        return v
+
 
 class ElevationDatasetAnalyzeResponse(BaseModel):
     dataset: ElevationDatasetSummary