ENH: _font: Set some defaults for the FontDescriptor class

This patch introduces some defaults for the FontDescriptor. This
might be useful if we would like to migrate existing code for
collecting font widths to the class, while not having all code
in place to also collect all associated other information about
the font.

The defaults I took from Google Gemini...

Author: PJBrs

pypdf/_font.py

@@ -1,4 +1,4 @@
-from dataclasses import dataclass
+from dataclasses import dataclass, field
 
 
 @dataclass(frozen=True)
@@ -8,16 +8,16 @@ class FontDescriptor:
     This contains both descriptive and metric information.
     """
 
-    name: str
-    family: str
-    weight: str
+    name: str = "Unknown"
+    family: str = "Unknown"
+    weight: str = "Unknown"
 
-    ascent: float
-    descent: float
-    cap_height: float
-    x_height: float
-    italic_angle: float
-    flags: int
-    bbox: tuple[float, float, float, float]
+    ascent: float = 800.0  # A typical ascent value for many fonts
+    descent: float = -200.0  # A typical descent value, negative
+    cap_height: float = 700.0  # A common cap height, often around 70% of the ascent
+    x_height: float = 500.0  # A common x-height, often around 50% of the ascent
+    italic_angle: float = 0.0  # Most fonts are not italic
+    flags: int = 4  # The "FixedPitch" bit is often set for default fonts
+    bbox: tuple[float, float, float, float] = field(default_factory=lambda: (0.0, -200.0, 1000.0, 800.0))
 
-    character_widths: dict[str, int]
+    character_widths: dict[str, int] = field(default_factory=dict)