Add bidirectional API types to frontend (#6867)

# Description of Changes
Fix https://github.com/Stirling-Tools/Stirling-PDF-SaaS/issues/281. Add
generated backend API mappings to the frontend code, and the logic to
convert from a backend API to frontend parameters objects.

Previously, it was impossible to tell if changing the backend API would
require a change to the frontend to support it because the frontend had
no static type information about the backend API. This PR adds
autogenerated tool API types to the frontend (in `toolApiTypes.ts`) and
adds explicit typed mappings between the frontend parameter types and
the backend API types, so theoretically the type checker should be able
to catch issues when changing one puts us in an invalid state with the
other. During development, it pointed out several inconsistencies that
we have between the frontend and backend types, some of which were
genuine bugs, and others were only happening to work because the backend
is more permissive than its API claims to be.

This also unlocks the ability for us to render the frontend settings on
saved backend API structures, which we've previously had to avoid doing
because we had no reverse mapping.

engine/src/stirling/models/tool_models.py

@@ -107,7 +107,7 @@ class KeyLength(IntEnum):
 
 
 class AddPasswordParams(ApiModel):
-    key_length: KeyLength = Field(..., description="The length of the encryption key")
+    key_length: KeyLength = Field(KeyLength.integer_256, description="The length of the encryption key")
     owner_password: SecretStr | None = Field(
         None,
         description="The owner password to be added to the PDF file (Restricts what can be done with the document once it is opened)",
@@ -1283,8 +1283,8 @@ class PageSize1(StrEnum):
 class SplitForPosterPrintParams(ApiModel):
     page_size: PageSize1 = Field(..., description="Target page size for output chunks (e.g., 'A4', 'Letter', 'A3')")
     right_to_left: bool = Field(False, description="Split right-to-left instead of left-to-right")
-    xfactor: int | None = None
-    yfactor: int | None = None
+    x_factor: int = Field(2, description="Horizontal decimation factor (how many columns to split into)", ge=1, le=10)
+    y_factor: int = Field(2, description="Vertical decimation factor (how many rows to split into)", ge=1, le=10)
 
 
 class SplitPagesParams(ApiModel):