Detect Nexus operation changes during replay

[Sushisource]

Replaces #1405

What

• compare the service and operation in NexusOperationScheduled history events with the command produced during replay
• report nondeterminism when either value changes
• gate the new checks behind a dedicated Core internal flag so existing histories remain compatible
• add regression coverage and an Unreleased changelog entry

Why

The Nexus operation state machine accepted scheduled history events without validating their service or operation. This allowed workflow code to change either field during replay without raising the non-determinism error used by comparable activity and child-workflow state machines.

Fixes #1231

Testing

• cargo test -p temporalio-sdk-core nexus_operation_state_machine::tests --lib
• cargo test -p temporalio-sdk-core --lib (367 passed, 1 ignored)
• cargo clippy -p temporalio-sdk-core --lib --tests --all-features -- -D warnings
• cargo fmt --all --check
• git diff --check