Use target serialization context for system Nexus signal-with-start

[tconley1428]

Summary

• resolve serialization-context factories from generated system-Nexus operation metadata
• retain the resolved context on the internal pending Nexus operation handle
• use that context while serializing the scheduled Nexus command
• add an end-to-end codec test proving signal-with-start nested payloads use the target workflow context

Why

Signal-with-start user payloads are delivered to the target workflow, so they must use the target workflow serialization context. Ordinary Nexus operations remain context-free.

Dependency

Depends on temporalio/nex-gen#73, which generates the operation registry serialization-context factory. The temporary SDK-core WIT edit used for local verification is intentionally not part of this PR; the directive will come from the API source update.

Closes #1645

Validation

• regenerated the system-Nexus binding with the local nex-gen binary
• ruff check and basedpyright: 0 errors, 0 warnings
• tests/nexus/test_temporal_system_nexus.py: 6 passed