Fix interceptor contract inconsistency for start_update_with_start_workflow

[brucearctor]

Summary

Fixes #1582.

StartWorkflowUpdateWithStartInput was the only OutboundInterceptor input dataclass missing top-level rpc_metadata and rpc_timeout fields, breaking interceptors that generically set metadata on every outbound call.

Additionally, _ClientImpl._start_workflow_update_with_start() in temporalio/client/_impl.py called execute_multi_operation() without passing metadata= or timeout=, silently dropping any RPC metadata.

Changes

temporalio/client/_interceptor.py

• Added rpc_metadata: Mapping[str, str | bytes] and rpc_timeout: timedelta | None fields to StartWorkflowUpdateWithStartInput
• Expanded the docstring to document that these top-level fields are authoritative for the gRPC call, while the sub-input fields exist for interceptor introspection only

temporalio/client/_client.py

• Populated the new fields at the construction site in Client._start_update_with_start()

temporalio/client/_impl.py

• Threaded rpc_metadata and rpc_timeout through _start_workflow_update_with_start() to the execute_multi_operation() gRPC call
• Also added retry=True to the execute_multi_operation() call to match every other gRPC call site in this file (this was a pre-existing omission, not part of [Bug] Interceptor contract inconsistency for OutboundInterceptor.start_update_with_start_workflow #1582)

tests/worker/test_update_with_start.py

• Added test_update_with_start_rpc_metadata_and_timeout_forwarded which verifies:
  • An interceptor can set rpc_metadata via the new top-level field
  • Caller-provided rpc_metadata is preserved
  • rpc_timeout is forwarded to the gRPC call

Backward Compatibility

• StartWorkflowUpdateWithStartInput is only constructed internally (in Client._start_update_with_start), not by external callers
• Existing interceptors (OpenTelemetry, LangSmith) don't touch rpc_metadata on this method — they manipulate headers. No changes needed to them
• The sub-input classes retain their own rpc_metadata fields for introspection

Testing

• All 33 tests in tests/worker/test_update_with_start.py pass
• pyright: 0 errors, 0 warnings
• ruff + pydocstyle: clean