fix: allow replacing worker client on default runtime

[Sakshamm-Goyal]

Fixes #657.

A replacement client with no explicit runtime uses Runtime.default(), but the worker compared the configured runtime value directly and treated None as a different runtime. Resolve the replacement client runtime with the same defaulting rule used during worker construction before comparing identities.

Regression coverage connects a second client without supplying runtime and verifies it can replace a worker client. The explicit different-runtime rejection remains covered.

Validated with:

• pytest tests/worker/test_workflow.py::test_workflow_replace_worker_client_default_runtime tests/worker/test_workflow.py::test_workflow_replace_worker_client_diff_runtimes_fail -q
• ruff format --check temporalio/worker/_worker.py tests/worker/test_workflow.py
• git diff --check

[Sakshamm-Goyal]

Addressed the runtime-selection concern in 8c313d8. The setter now compares only an explicitly configured runtime; it no longer creates Runtime.default() merely to validate a replacement client with no configured runtime. Explicit mismatches still raise.

The regression now creates the worker with a non-default runtime and replaces its client with one that has no runtime argument, which failed under the previous check. Source compilation and focused Mypy passed. I attempted the targeted pytest path, but this worktree's SDK Core submodule is absent, so its bridge extension cannot be built locally; fresh CI is queued for the full pinned-environment test.