fix: preserve restricted proxies after in-place operations

[Sakshamm-Goyal]

What changed

Fixes #772. In-place operations on a sandbox-restricted mutable object now keep the restriction proxy instead of replacing it with the raw object.

Why

_RestrictedProxyIOp built a wrapper that returned the proxy, but stored it under bind_f instead of the lookup descriptor's bind_func. The normal binding path therefore returned the underlying object after +=, bypassing the proxy for later accesses.

Validation

• Added a regression covering list += [...] through _RestrictedProxy.
• pytest tests/worker/workflow_sandbox/test_restrictions.py -q (4 passed)
• ruff check and ruff format --check on changed files
• pyright tests/worker/workflow_sandbox/test_restrictions.py

[Sakshamm-Goyal]

Fixed the CI mypy regression in 763e26c. The functional change correctly assigns the in-place-operation binder to bind_func, but _RestrictedProxyLookup did not declare that instance attribute, so Mypy inferred an incompatible optional assignment in the subclass.

The fix declares the binder's callable shape on the lookup class; it does not change runtime dispatch. The focused type check (mypy --follow-imports=skip temporalio/worker/workflow_sandbox/_restrictions.py) now passes with no issues. The behavioral regression test is already included in the PR; this local worktree no longer has its compiled bridge extension after UV resynchronization, so I did not report a false local pytest result. Fresh CI will validate the complete pinned environment.