Allow replacing a worker's client when both use the default runtime

[wuchris-ch]

Fixes #657.

The bug: If you swap a worker's client (worker.client = new_client) and neither client was given a runtime, it fails with "New client is not on the same runtime as the existing client". But both clients are acutally on the same default runtime.

Why: A client with no runtime falls back to Runtime.default(). The check compared against the client's configured runtime, which is None, so it never matched.

The fix:

• The service client now remembers the runtime it actually connected with.
• The worker uses that when it's created and when its client is swapped.
• The check doesn't create a default runtime just to compare. No recorded runtime means no match.
• This is the second option suggested in [Bug] Cannot replace worker client if default Runtime is used #657, so the client's config stays as the user set it.

Still rejected (the case from the #1707 review): a worker on a custom runtime plus a new client with no runtime. That client is on the default runtime, so it's a real mismatch.

Tests:

• test_workflow_replace_worker_client_implicit_default_runtime: fails on main with the error above, passes with this change.
• test_workflow_replace_worker_client_implicit_default_runtime_mismatch_fail: the custom runtime case.
• Ran locally: poe build-develop, poe lint and poe test (1446 passed, 9 skipped). The new tests also pass with --workflow-environment time-skipping.
• Changelog: changelog/fixed/relay-racing-raccoon.md (changelog-tool check passes).

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