Manage snapshots mutability issue (#2431)

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Related to #2409, and partially closes #2427

# Rationale for this change

This PR fixes a thread safety issue in the `ManageSnapshots` class
similar to the one identified in `ExpireSnapshots` (#2409). While the
original issue specifically mentioned `ExpireSnapshots`, the same thread
safety vulnerability exists in `ManageSnapshots` due to identical
problematic design patterns. The same testing methodology used in #2430
was adapted for this.

**Root Cause:** 
The `ManageSnapshots` class had class-level attributes (`_updates`,
`_requirements`) that were shared across all instances. When multiple
threads created different `ManageSnapshots` instances for concurrent
operations, they all shared the same underlying tuple objects for
tracking updates and requirements.

**Potential Impact:**
- Thread 1: `table1.manage_snapshots().create_tag(...)` adds updates to
shared tuple
- Thread 2: `table2.manage_snapshots().create_branch(...)` adds updates
to same shared tuple
- Result: Both threads would have mixed updates, potentially causing
incorrect operations or failures

**Solution:**
Applied the same fix as ExpireSnapshots - moved the shared class-level
attributes to instance-level attributes in the `__init__` method,
ensuring each `ManageSnapshots` instance has its own isolated state.

**Relationship to #2409:**
While #2409 specifically reported ExpireSnapshots thread safety issues,
this PR proactively addresses the same vulnerability pattern in
ManageSnapshots to prevent similar issues from occurring with snapshot
management operations (tags, branches, etc.).

## Are these changes tested?

> 📢 🔥 Big shout-out to @QlikFrederic, as the testing methodology was
largely derived from the testing and analysis done by the user! 🔥 📢

Yes, comprehensive test coverage has been added with a dedicated test
file `test_manage_snapshots_thread_safety.py`:

- **`test_manage_snapshots_thread_safety_fix()`** - Verifies that
different ManageSnapshots instances have separate update/requirement
tuples
- **`test_manage_snapshots_concurrent_operations()`** - Tests concurrent
operations don't contaminate each other
- **`test_manage_snapshots_concurrent_different_tables()`** - Tests
concurrent operations on different tables work correctly
- **`test_manage_snapshots_cross_table_isolation()`** - Validates no
cross-contamination of operations between tables
-
**`test_manage_snapshots_concurrent_same_table_different_operations()`**
- Tests concurrent operations on the same table

All tests demonstrate that the thread safety fix works correctly and
that concurrent ManageSnapshots operations maintain proper isolation.

## Are there any user-facing changes?

**No breaking changes.** The public API remains identical:
- All existing `ManageSnapshots` methods work the same way
(`create_tag`, `create_branch`, `delete_tag`, etc.)
- Method signatures are unchanged
- Behavior is identical except for the thread safety improvement

**Behavioral improvement:**
- Concurrent `manage_snapshots()` operations on different tables now
work correctly without interference
- No risk of mixed updates/requirements between different
ManageSnapshots instances in multi-threaded environments
- Improved reliability for applications using ManageSnapshots in
concurrent scenarios

This is a pure bug fix.

Author: ForeverAngry

pyiceberg/table/update/snapshot.py

@@ -810,8 +810,13 @@ class ManageSnapshots(UpdateTableMetadata["ManageSnapshots"]):
        ms.create_tag(snapshot_id1, "Tag_A").create_tag(snapshot_id2, "Tag_B")
     """
 
-    _updates: Tuple[TableUpdate, ...] = ()
-    _requirements: Tuple[TableRequirement, ...] = ()
+    _updates: Tuple[TableUpdate, ...]
+    _requirements: Tuple[TableRequirement, ...]
+
+    def __init__(self, transaction: Transaction) -> None:
+        super().__init__(transaction)
+        self._updates = ()
+        self._requirements = ()
 
     def _commit(self) -> UpdatesAndRequirements:
         """Apply the pending changes and commit."""