Make Metadata class readonly/immutable

Add frozen=True to the Metadata dataclass to make all attributes
readonly after creation. This prevents accidental modification of
metadata that represents the immutable properties of a database file.

Changes:
- Added frozen=True to @DataClass decorator
- Updated HISTORY.rst to document the breaking change
- Noted that C extension Metadata has always been readonly

Benefits:
- Consistent behavior between pure Python and C extension
- Prevents bugs from accidental metadata modification
- Better represents the immutable nature of database metadata

Breaking change: Attempting to modify Metadata attributes after
creation will now raise an AttributeError. This brings the pure
Python implementation into consistency with the C extension, which
has always had readonly attributes.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

Author: oschwald

HISTORY.rst

@@ -23,12 +23,14 @@ History
   instances, and future-proofing for Python 3.14's InterpreterPoolExecutor.
   Requested by R. Christian McDonald in GitHub #105.
 * **BREAKING**: The pure Python ``maxminddb.reader.Metadata`` class has been
-  converted to a dataclass. The ``__repr__`` format has changed from
-  ``maxminddb.reader.Metadata(...)`` to ``Metadata(...)``. If you depend on
-  the exact string representation of the Metadata object, you will need to
-  update your code. All functionality remains the same, including the
-  ``node_byte_size`` and ``search_tree_size`` properties. The C extension's
-  Metadata class is unchanged.
+  converted to a frozen dataclass. The ``__repr__`` format has changed from
+  ``maxminddb.reader.Metadata(...)`` to ``Metadata(...)``. More importantly,
+  all Metadata attributes are now readonly and cannot be modified after
+  creation. If you were modifying metadata attributes after object creation,
+  you will need to update your code. All functionality remains the same,
+  including the ``node_byte_size`` and ``search_tree_size`` properties. Note:
+  The C extension's Metadata class has always been readonly, so this change
+  brings the pure Python implementation into consistency with the C extension.
 
 2.8.2 (2025-07-25)
 ++++++++++++++++++

maxminddb/reader.py

@@ -308,7 +308,7 @@ def __enter__(self) -> Self:
         return self
 
 
-@dataclass(kw_only=True)
+@dataclass(kw_only=True, frozen=True)
 class Metadata:
     """Metadata for the MaxMind DB reader."""