models: Split PdfSignatureConfiguration into separate types

- Introduced `PdfNewSignatureConfiguration` and `PdfExistingSignatureConfiguration`
  to replace the unified `PdfSignatureConfiguration` type.
- `PdfSignatureConfiguration` is now a union of `PdfNewSignatureConfiguration`
  and `PdfExistingSignatureConfiguration`, allowing stricter type validation
  and clarity.
- Ensures each signature type has its respective required and optional fields.

Assisted-by: Codex

Author: datalogics-kam

src/pdfrest/types/public.py

@@ -161,14 +161,27 @@ class PdfSignatureDisplay(TypedDict, total=False):
     reason: str
 
 
-class PdfSignatureConfiguration(TypedDict, total=False):
-    type: Required[Literal["new", "existing"]]
+class PdfNewSignatureConfiguration(TypedDict, total=False):
+    type: Required[Literal["new"]]
     location: Required[PdfSignatureLocation]
     name: str
     logo_opacity: float
     display: PdfSignatureDisplay
 
 
+class PdfExistingSignatureConfiguration(TypedDict, total=False):
+    type: Required[Literal["existing"]]
+    location: PdfSignatureLocation
+    name: str
+    logo_opacity: float
+    display: PdfSignatureDisplay
+
+
+PdfSignatureConfiguration = (
+    PdfNewSignatureConfiguration | PdfExistingSignatureConfiguration
+)
+
+
 class PdfPfxCredentials(TypedDict):
     pfx: Required[PdfRestFile]
     passphrase: Required[PdfRestFile]