Handle truncated pack deltas

[ctoth]

Handle truncated pack deltas

Hi! This continues the Hypothesis property-test work for Dulwich.

Summary:

• add pack delta round-trip properties for create_delta and _create_delta_py
• add a malformed-delta property that checks parser failures use ApplyDeltaError
• fix Rust apply_delta bounds checks so truncated copy/insert op streams do not panic
• run property tests with unittest discovery so new property modules are included

The malformed-delta property found a real panic in the Rust extension. A minimal
reproducer was:

apply_delta(b"", b"\x00\x01\x01")

Before this change, that could panic inside the Rust slice copy path for a
truncated insert operation. After this change, it raises ApplyDeltaError, which
matches the Python-facing error contract used for malformed deltas.